Disk Brake Differences Cadilliac Rear Disk Brakes PARKING BRAKE PULLEYS' Brake Analysis

By Dave Mumert

Don't get too excited about wider drums and bigger pads.

The formula for brake torque on a disk brake system is
Torque = (Brake Pressure) X (Piston Area) X (Effective Rotor Radius) X (Pad Coefficient of friction) X 2
Notice there is no mention of pad area. The coefficient of friction (u) is measured in force per square inch per pound of pressure

So a 80 mm caliper (about 7.8 square inches) with 1000psi of brake pressure will put 7800 pounds of force on each pad.
A 10 square inch pad will have 7800/10=780 pounds per square inch - so force will be 780 X u X 10 square inches or 7800u
A 12 square inch pad will have 7800/12=650 pounds per square inch - so force will be 650 X u X 12 square inches or 7800u

Larger pads should last longer and may fade less but they don't provide more brake torque. The same applies to wider shoes on the rear.
You can get more braking by:
1 - Increasing brake pressure - not easy without changing the booster
2 - Increasing pad friction - pretty easy
3 - Increasing rotor diameter - very difficult
4 - Increasing caliper piston size.

Going to 80mm calipers and rear disks may force you to use a larger bore master cylinder which will reduce your available brake pressure.

Here are the data for various Master Cylinder and Caliper sizes


Here is the actual spread sheet so that you can vary some of the assumptions
(load this file and run it in Excel)

If you are serious about brakes you should get some reading material and be sure you understand what is involved.

I'm with you on this, the rear brakes need some help.  I would look at sticky pads (see Jim K) and maybe a hydra boost system.  The system to get rid of the brake torque from the bogie arms looks like a good place to start. (see Jim K)Dave  M


Brake Systems Analysis by Frank Condos



J.R. Wright

1 70MM calipers  drums with asbestos shoes with standard 15/16" slave cylinders
2 80MM calipers  drums with asbestos shoes with standard 15/16" slave cylinders
3 70MM calipers  drums with Carbon Metallic shoes
4 80MM calipers  drums with Carbon Metallic shoes with 1 1/8" slave on middle bogey/15/16"slave on the back
5 70MM calipers  drums with asbestos shoes on the rear bogey with 15/16" slaves,  Disk brakes with asbestos or carbon metallic pads on the middle bogey
6 80MM calipers  drums with asbestos shoes on the rear bogey with 15/16" slaves,  Disk brakes with asbestos or carbon metallic pads on the middle bogey
7 80MM calipers  Disk brake package installed on both rear bogeys with asbestos or carbon metallic pads

7.  This setup uses several combos of Calipers on the back bogey, one would be using the Cadillac 70mm with the mechanical parking brake or the 70mm calipers from the front of the coach or from the 1984 BOP cars.  By the way BOP means Buick/Oldsmobile/Pontiac.  An 80MM is used on the middle bogey and is the same unit that was used on the front brakes.

The disk brakes could be the Cadillac Eldo system, Harrison brake system, TSM or others

As for master cylinders the stock unit will work with most applications.  The installations with the larger slave cylinders and calipers would benefit from the use of a Chevy P-30 master cylinder that has more volume.

This list is by all means is not complete and many other parts have been used to modify and update the GMC's braking systems.'

You need to understand what you need before you start.  Most people  that have done the cad rear brake conversion do not use the cad  calipers with the emergency brake feature.  It has a tendency not to  release and the rear brake stays locked.  The base set up is the 80MM
calipers on the front and middle bogey and the 70MM on the rear.  The  70mm are the same unit that the front uses as stock.  As for an
emergency brake some use a line lock valve that you set when you push  down on the brakes.  I recommend the carbon metalic pads on all

The second shortcut has a parts list near the end.

If you decide that you what to use the cad rear calipers with the brake  lever you must find and old set to use as a trade in because you need
the spring and arm assembly to hitch up the cable.  The rebuilt units  do not come with the spring and arm assembly and they are difficult to
buy. Take a good look at all the pictures. J.R. Wright

I promised information about the pre- and post-78 brake caliper dimensions.These came from the 78 Eldorado calipers I've got off of the rear of my coach and from the 80 Monte Carlo calipers I'm about to install, eyeballedwith my 6" vernier caliper:

                 Mounting Piston
                 Width    Diameter
78 Eldorado:     5.355"   2.98"
82 Monte Carlo:  3.75"    2.35" (60mm)

Ken Henderson

The biggest problem doing the installation seems to be the necessity for modifying the actuator (pedal & cross-shaft).  The Hydroboost extends much farther into the cockpit than does the OEM booster.  That calls for moving the cross-shaft aft (where the OEM dash interferes), and/or extending the bellcrank.  And the HB must be mounted lower on the firewall to make room for the MC to clear the front "hood". GMCwiperman kenH

A diesel PS reservoir is desireable because it's got a second fluid return line, but it's not essential; most have no trouble with a tee in the single return.

The P-30 MC fits just fine on the HB but it may have to mounted a little lower that the OEM would require since its reservoir is taller. ORSIT Ken H.

I had the HB custom made for me, there are at least 10 or 12 ways to build one up, depending on the application and how much stopping power is needed. Too lengthly to explain here, but you need a large power piston and the small reaction piston to give you the pressure with the least amount of pedal pressure to do the job right for the MH. The one off the Astro van is NOT the one you want on the MH, it is not set up correctly for the MH. I did a Power Point at one of the GMCMI conventions on my installation.

You can view some pics here;
Make sure you have a good power steering pump, as that is necessary to furnish the needed hydraulic pressure for the Hydro-boost. The OEM Saginaw P2 pump for the MH will be OK, you can test the output pressure by the instructions in the MH manual. Pressure from the pump is supplied to the hydroboost, and then on to the PS unit. I would recommend two returns on the pump canister, as you do not want to have interference with the return fluid on the spool valves. The early to mid 80's vehicles with the 350 Diesel had these, if you have a good pump, just switch the canister to your good unit, or braze a return nipple on the canister you have.

I worked with Tim at Precision Rebuilders in St Clair, MO to build up my unit.

This is what I have in my unit;
Input (reaction) rod size= .545"
Power piston diameter= 1.56"
Lever ratio of .72 to 1
Accumulator volume= 3.0 cu in
Use the shortest pedal rod you can get.

The unit comes into the cockpit area further than the vacumnn booster, approx 1-5/8" more, but that was no problem for me with a custom made dash. You can do the math on changing the pedal ratio, I changed mine from the 4 to 1 to 5.5 to 1.

Make sure all your braking components are in excellent shape, as, if done correctly, there will be increased brake hydraulic pressures. BOBD
A couple of weeks ago I tore down a Hydroboost just for curiosity. Pictures are at if you're
curious also. Ken Henderson
A note of caution, it may be necessary to replace all brake lines. The Powermaster (and Hydroboost) delivers considerably more line pressure to the system than the stock MC and vacuum booster, so your lines must be in excellent condition. Failure to replace rusted or otherwise compromised lines could result in lines bursting under pressure, possibly when you need them the most. JWID Larry  :)


I noticed on their web page an interesting collection of center brake lights.(purchashed from Scott Nehoda see below)  I picked on that mounted on the surface, looked reasonably stock, and that was a sealed LED unit.

Installing it was pretty easy. I also got the brake light module they sell that solves the problem of not having a clean brake-light circuit in the back. This module will light the center brake light at any time both turn circuits are energized, and then latch it on until both turn circuits are de-energized. It works like a charm.

Here are pictures:

Rick "who gives himself an easy project to check off once in a while as an antidote to major projects" Denney
Glad it worked out so well for you. I now have the controller module available- The module is able to handle up to 3amps---LED is best but incandesant lights could be used.
Scott Nehoda

Here are pictures of a cool third brake light from an Escalade that looks great on the GMC
Al scott


As most all of us have changed the master cylinder, you would have a spare lid. I braised on a nipple to feed fluid under pressure from a can. You don't need much pressure. If you have too much, the top will leak. I have that top to loan , if anyone wants to use it. Dean Hanson

This note came through over the GMCnet and no one commented.  Yet it is one of the great ideas of the year.  GMCs are known to be difficult to bleed the brakes.  The combination of Gravity and Brake Peddle seem to be the most successful but requires a lot of patience and is a two person job.  Pressure bleeding has been used for years by manufacturing and truck and brake shops.  The problem is getting the proper equipment.  This is a low cost way to build your own Pressure Brake bleeding system.  The really keen part of this tool, is the siphon to return the fluid to normal level after filling.

This is especially useful when you want to flush all the fluid in the whole system.  It takes a lot of fluid and a lot pumping if you are using the brake peddle.

Click here to read about:
The Making of the Inexpensive Brake Pressure Bleeder

I would add only one feature.  There used to be a kit that was called Easy Bleed.  He used a spare tire (could be one of those Mickey Mouse skinny spares) for the air source.  This has the advantage of a large volume of air at a constant pressure that you can set with your air compressor.  The tire will have enough air to do the whole rig. Gene
One advantage of pressure bleeding is that master cylinders often seem to have a tendency to fail soon after brakes are bled bny pumping the pedal.  That is due to people pushing them all the way down to the floor when opening the bleeder valves.  This allows a portion of the master cylinder seals to ride farther down into the bore than they normally would.  Sometimes there is surface residue on the bores in this area that can scuff the seals and then they will subsequently begin to leak.

If one is going to bleed by having someone pump the brake pedal, it is a good practice to put a 3/4" board on the floorboard under the pedal so that the pedal can never go all the way to the floor. Emery


I went to a junk yard and got a MC cover off of an old GMC truck with the same type of cover.  Bought two cover gaskets from NAPA ....came off of their "HELP" display. You need two gaskets to get the cover to seal under the 10lbs of pressure provided by the insect/weed sprayer. You must cut the center out of the two sides of the two gaskets, drill two holes in the top of the recycled cover for brass fittings to accept the hose from the sprayer, screw some self tapping sheetmetal screws  with silicone gasket cement on them into the vent holes in the cover, and seal the inside of the cover around the brass fittings with silicone.  Then you need a "Y" or "T" fitting to split the
fluid to both sides of the MC, and put screw clamps on the hose connections.   Then put about 2 quarts of fluid in the sprayer, bleed the lines going to the MC cover, clamp the cover on the MC, pump it up to 10 lbs. and start bleeding.  It works GREAT!!  Only need one person to do it.
10 lbs is enough to expand the pistons in the calipers and WC out to make contact with the discs and drums.

I used Super Blue Racing Brake Fluid.

Color of the fluid is Blue, so that as you bleed, when it turns to blue, you know that the old stuff has passed out. Building the Bleeder was the hardest part of the old hoses came off  easy....lucky me!! Larry  :-)

80 MM Front Brake Caliper Upgrade

80 mm Caliper numbersJim Wagner suggested the 84 1/2t 2wd Suburban with JB7, JD7 brakes.
--------------Parts needed to change over to 80 mm caliper's on the front wheels are Autozone and are as follows:

        2 ea    Banjo bolt, p/n 313940, $1.99
        1 ea    Caliper, left side, p/n 90185, $12.99 + core charge ($17.00)
        1 ea     right side, p/n 90184, price same as above
        4 ea     Caliper mounting bolt, p/n H5004, $1.29
        1 ea     Brake pad set, p/n 1534, $37.99 (these are Performance Friction carbon metallic pads)
        1 ea     Flexible hose, left side, p/n 77320, $24.99,Wagner F110424,NAPA 38563-702
        1 ea     right side, p/n 77421, price same as above,Wagner  F110425,NAPA 36954-726
                For the brake hoses, try Raybestos # BH38563 & BH36954. You will need two
                5/8 national fine jam nuts to attach the hoses to the bracket at the frame.
                You will have to slightly alter and move the bracket that is in the middle
                of the hose. The steel tube end that fastens to the caliper will have to be
                bent a little to clear the upper ball joint.Denny
        2 ea     bracket clip, p/n H1457, $1.29
        2 ea     jam nut (don't have AutoZone number for this)
        2 ea     bolt and nut, ss, ¼ x 1" (not an AutoZone item)

One of the benefits of going with an 80 mm caliper is it applies, I'm told, approximately 1000 psi more braking force vs. original caliper.  The difference is noticeable.

Although there was some degree of controversy on this subject a number of months back, you may want to consider going to a larger master cylinder if making the switch to larger the calipers.  If so, it's available from
Autozone, p/n 10166, $53.43, and does require an ~ 0.010" longer brake rod (p/n GM 5469384)(between master cylinder and brake booster) as well as a modified mounting bracket.  If you need further details on this
modification, let me know.   Paul Bartz

80 mm Front Brake Caliper Upgrade

Round Gauge Cluster Brake Light

  The brake light that comes on in the lower right of the round Gauge Cluster  (fuel gauge at top) is not parking brake related but it is truly a brake  warning light and should not be ignored.  It is triggered by the proportional  valve located on the left frame behind and above the driver's side front  wheel.  The proportional valve has brake lines going into it from the master  cylinder and out to the front and rear calipers and wheel cylinders.  There  is an internal sliding valve that should normally be centered.  If the  pressure is lower in the front brake circuit the valve slide forward and the  switch grounds the wire coming into the top of it.  The same thing happens  when the back brake circuit has lower pressure.  Marlene -- try pulling the  wire clip off the top of the proportional valve (turn on your key) and with  it disconnected the brake light should go out.  When you ground it to the  frame or other ground it should light.  It may be that you've got a short in  the wire somewhere or you might actually have a brake problem.  A sticking  wheel cylinder could cause this.  A faulty proportional valve could also be  the problem. Internal leak in the master cylinder or just air in the system  somewhere.  If you have never replaced the front hoses you should try that  first.  They have a history of collapsing internally.  This could cause a  higher pressure in the front brake circuit and trigger the light.  Bleed the  master cylinder and then all the wheel lines.
Emery Stora

Stainless Steel Brake Lines

Classic Tube 1-800-882-3711 Talk to Tim Slattery, FAX 1-716-759-1014


Disk Brake Differences

Leigh Harrison
 - uses a huge rotor on the center axle(15 inch<?>) which requires a
   backspacer to move the rotor inboard
 - complete "bolt-on" kit
 - all components cad plated
 - Expensive(quality costs)
 - Requires P30 master cylinder

 - Less expensive
 - Works with original master cylinder
- Not really a "bolt-on" - if you don't get the optional braided rear
   hoses you must come up with an acceptable way to plumb the calipers
   and the braided hoses are not that great either.
 - Currently limits you to a 60mm caliper
 I suggest that you look at Bill Harvey's write up on the GMC Western States web site:

Also check out Heinz Wittenbecher's complete instructions on the TSM system at:

Heinz later changes to the Harrison system.  You can see info on that at:

Heinz points out that there was nothing wrong with the TSM system but since the Harrison system uses larger calipers he wanted to try them. One of the big differences is cost.

I have had the TSM system in for about 2 years now and am very pleased with it.  I have towed my CJ-7 Jeep (over 4000 pounds of tow weight) up and down many mountain grades and the disk brakes perform extremely well.  I don't have an aux. braking system on my toad and I had experienced brake fade and vapor lock with the drum brake system even while using the asbestos brake shoes that I had bought from Cinnabar and I also had to adjust the brakes frequently due to shoe wear, but I have had absolutely no problems once I switched to the disk brakes.

When I installed the disks I had a hard time bleeding the rear brakes.  It turned out to be a capacity problem with the master cylinder.  The four calipers just take a lot more fluid than four stock wheel cylinders.  My solution was to put on a larger master cylinder as shown on my PhotoPoint site:
After installing the larger master cylinder they rears bled easily.

Cost was definitely a factor in my decision to use the TSM brake system.  I used their Cadillac rear calipers with the parking brake levers since I wanted to use my stock parking brake lever.  This is a slightly smaller caliper than the mid axle ones but they seem more than adequate for stopping the GMC even when pulling a heavy toad.  I modified the parking brake cable system to use pulleys instead of the wire guides that cause a lot of drag on the cable.  If anyone needs details on those mods, let me know.

Jim Anstett has done an excellent job on getting the stock drum system to work about as well as it can, but I just feel that the advantages of disk brakes are really great.  Easy to check the pad thickness just by looking at them without disassembling anything, very easy to quickly replace the pads when needed, no need to periodically adjust the rear brakes, and no fade or vapor lock due to the cooling running inherent in the design of disk brakes.  It is no accident that manufacturers install all wheel disk brakes on modern, high tech vehicles.Emery Stora
76-78 Eldorado rear disk brakes
 Another alternative is the "roll your own" using 76-78 Eldorado rear disk brakes.

Cadilliac Rear Disk Brakes Modification

J. R. Wright

I have uploaded the pictures for the Cadillac Disk brakes modification for the coach.  These were done at the NF rally and I have included the hand out
that was also available.

        Combination Valve (Brass)       GM #25509419

Front brakes
        Front Calipers  OEM (not over size)
        Brake Pads      Carbon Metallic (Performance Friction Co)  P/N  0524  (about $40)
        New Hoses       ????? Long Toronado (2) BH36675
        Rotors can be obtained from Cinnabar for $125 each 1-800-720-2227

Rear Brakes
        Mid Axle Cyl    1 1/16"  Wagner F79767
        Rear Axle Cyl   15/16"   Wagner F51081 OEM Size
2 sets  Brake Shoes     Asbestos Only   GM #8020290/Delco -- BUY FROM GATEWAY--
        Springs                         #171-500


   P-30 master cylinder
      Autozone, p/n 101668, $53.43, and does require an ~ 0.010" longer brake rod
      (p/n GM 5469384)(between master cylinder and brake booster) as well as a
      modified mounting bracket.
      Wagner P/n F79821      $60

        Parts, from Autozone front wheels are:

        2 ea Banjo bolt,                  p/n 313935, $1.89
        1 ea Caliper, left side,          p/n 90185 , $12.99 + core charge($17.00)
        1 ea Caliper, right side,         p/n 90184 , price same as above
        4 ea Caliper, mounting bolt,      p/n H5002 , $3.39
        1 ea Brake pad set,               p/n 1534  , $37.99 (these are Performance Friction carbon metallic pads)
        1 ea Flexible hose, left side,    p/n 77320 , $24.99
        1 ea Flexible hose, right side,   p/n 77421 , price same as above
        2 ea Flexible hose, bracket clip, p/n H1457 , $1.29
        2 ea Flexible hose, jam nut (don't have AutoZone number for this)
        2 ea bolt and nut, SS, ¼ x 1" (not an AutoZone item)


   Eldo proportioning valve between mid and rear axles. (Approx $35.00)

.... 76-78 Cadillac El Dorado rear wheel backing plates ....
   One option for converting to rear disc brakes is using 76-78 Cadillac El Dorado rear wheel backing plates
   as the basis to mount the caliper's and elimination of the dust shield on the backing plate is required,
   as well as a slight notching of the backing plate to clear the swing arm. The backing plate mounting bolt
   holes do not need to be enlarged or moved. Machining is only required to allow the backing plate to fit over
   the axle shaft by enlarging the center hole (2.250" with a 45 degree by 1/16" chamfer on the back side).

                       *** CAUTION ***
   *** The Front Bogie Backing plates NEED to be strengthened/Reinforced. ***
   *** Especially if 80mm calipers are used. Lots of torque here. ***
   The back bogie can use the stock plates/calipers w/parking brake.

   The rotor requires locating and drilling three new holes (same size as existing) for the mounting bolts.

   Additional parts, from Autozone  rear disc brakes on the rear wheels are:
      2 ea Banjo bolt,                      p/n 313940,   $1.99
      1 ea Caliper, left front rear axle,   p/n 90185,    $12.99 + core charge ($17.00)
      1 ea Caliper, right front rear axle,  p/n 90184,    $12.99 + core charge ($17.00)
      1 ea Caliper, left rear rear axle,    p/n 90557,    $73.94 + core charge ($70.00)
      1 ea Caliper, right rear rear axle,   p/n 90553,    $73.94 + core charge ($70.00)
      1 ea caliper bolt                     p/n H5004  @  $1.29
      8 ea mounting bolt,                   p/n H5002,    $3.39
      1 ea Brake pad set (front rear axle), p/n 0524,     $26.99 (these are Performance Friction carbon metallic pads)
      1 ea Brake pad set (rear rear axle),  p/n 25265,    $6.49 (these are not Performance Friction carbon metallic pads)
      4 ea Flexible hose,                   p/n 77226,    $9.99
      2 ea bracket, left side               p/n 3757443,  $8.20
      2 ea bracket, right side,             p/n 10257203, $8.20
      4 ea bracket clip,                    p/n H1457,    $1.29
      4 ea jam nut (don't have AutoZone number for this)
      4 ea Brake line, 3/16 x 20",          p/n H320,     $2.49

      4 ea rotor.                           p/n 5512,     $36.94.

Emergency Brake...
   McMaster-Carr (  pulleys.
      P/N 3099T52 and they sell for $5.36 Total shipping was $2.17.


Parking Brake Improvement

Click for Detail A very simple and inexpensive improvement to
the brake cable system did improve the holding power of my brakes.  I replaced the
hooks that hold the cable where it passes through the frame (both sides) with  heavy
duty pulleys bolted to the clips that the hooks were attached to. I un-did the
connection on both sides where the cable attaches to the "U" near the rear wheels,
passed the cable through the pulley and bolted the cable back in place ----- just
enough cable to still make the connection. This eliminates a lot of the friction in the
emergency brake system.  My coach now holds on my driveway and all of you who have
visited will agree that it is STEEP.  If any one is interested I have  a source for
the "perfect" pulley ($12 for the pair). Gary

   Let me add a few things that I have been asked on this subject:

1)  The "special pulleys" are special only in the sense that they fit and are rugged.
The ones I used came from a logging supply store and were marked "Block Division,
Wichta Falls, Tx,  525#".  They are 3" from the center of the mounting eye to the extremity
of the pulley wheel -- just the right size to mount on the existing bracket.
They are galvanized laminated construction.  Now available from Scott            Ahohen Supply Co.

2)  The OEM installation of the brake cable had a hook rod on the interior of the passenger
side frame to route the cable away from the OEM muffler. I have a new muffler system and
this rod is not needed so I removed it.  If you need to keep the rod in order to clear the muffler
then a third pulley can be hung on the rod if it is shortened a few inches.  This will eliminate friction at this point.

3)  I think that the "emergency brake" should be more properly called a "parking brake".
Mine does hold on a fairly steep incline but I have no trouble overpowering it with my 455
and I sure would hate to have to depend on it to stop in an emergency situation.

4)  The addition of a vacuum pump to the brake booster is a must in my books.


Enhanced Booster

Product description by  Leigh Harrison (see link below)

Benefits: Great performance enhancer to the Brake Master Cylinder Kit /
 Old Vacuum boosters required 75lbs pedal pressure to activate valve.
Sensitized Booster initiates valve at 15lbs, and is fully activated at 25lbs pedal pressure.
No sending back your old booster for push rod.



(Kills a Vacuum tank Idea)

Well, I might have known, Gene & Emery are RIGHT!  Another of those ASSumptions, or believing something heard!

I connected a vacuum gauge to the accessory port on my vacuum reservoir. Cranked the engine and got 20"Hg.  Shut the engine down and waited about 3 minutes -- still had 20"Hg.  Put my foot on the brake.  The vacuum dropped about 2"Hg immediately, then continued to drop at about 0.5"Hg per second! Mind you, that's with the 4"x30" reservoir!  Without it the rate would be much higher.

Among the variations of the test I conducted was to run the engine, put my foot on the brake, and turn the Cadillac suspension compressor/vacuum pump on.  The pump WAS able to hold 20"Hg vacuum and to recover it if I cycled the brake.

With neither the engine nor the pump running, the pedal rose slowly, almost imperceptibly, as the vacuum decreased.  I was, frankly, surprised on the first trial, to find that it had moved:  I thought my foot was at the same level as when I first applied the brakes, until I released and reapplied the pedal and found that it went down only a little way.

I'm disappointed and baffled by this situation.  For all my power brake driving years -- at least 50 of them -- I've thought the vacuum was trapped with the pedal down and would hold the brakes indefinitely.  'Tain't so!

The need for the reservoir is now more acute than I had any idea -- that controlled leak means that the fabled 2-3 brake applications must be quick, short ones because one long one will deplete the vacuum.  And "long" will be pretty short.

An electrically driven pump also makes even more sense than I realized -- that's the only way to extend the boosted brake application time beyond about 30 seconds! :-(

Unless all of us have defective boosters.  KenH

Sensitized  so that it takes less peddle pressure to get full braking.
LAMEY ENTERPRISES Ontario, CA - (909) 983-7872(909) 982-7747
$175 plus shipping,  exchange or a core charge of $100

Carolina Classic Trucks

Applied GMC

Leigh Harrison


Master Cylinder:

mfg.    Wagner
P/N     F79821
Size    1 1/4" piston
OEM     1970+ Chevy  1 ton Truck
Cost    $60 new
Warranty        Lifetime  (Keep receipt)

If your parts supplier can not get a Wagner, then he can cross reference to local brand.  Don't get stuck with a rebuilt one, not worth taking home & small difference if bought Right.  This is a perfect fit for all '73 thru '78. It's best to bleed the master cylinder on the bench before
installation.  Better chance of getting all the air out of the fluid. My Wagner has been on my coach for approximately 9 years w/o any problems. Coach stops good with OEM brakes & with Brake Guard.
The part # for the new master cylinder for standard brakes is (MC39075) (NAPA #39705 ).  That is a Raybestos number and bolts right on.  I sell them for $65 no exchange, they are new.  You can use your old one as a wheel chock!Jim Bounds

Sinking Pedal Syndrome
I have not experienced the "Sinking Pedal Syndrome" but have heard of
several GMCers that have.  It's not a bad master cylinder typically, but
a bad procedure of bleeding the air out of the master cylinder before
installation (extensive bench bleeding required).  What happens when the
air is not completely removed from the master cylinder is that the air
will slip past the piston in the master cylinder & the pedal will slowly
go to the floor.  After  master cylinder removal & an extensive bench
bleeding,  all is well without exchange of the master cylinder.
I always have done "Bench Bleeding" of a master prior to installation &
therefore never seen this Syndrome.  Next time try to get all the bubbles
(even the tiny ones) out of the master cylinder prior to installation &
you will find gone is the Syndrome.
The procedure to determine if you have the "Sinking Pedal Syndrome" is to
start the engine (Vacuum Booster operating) & press hard on the brake
pedal for 60 seconds or so & if the pedal starts to slowly sink then you
have it.
Power of the "Net" illustrated again !!!!. Duane Simmons


During October 2006, in an effort to find a master cylinder that worked better with my 80 mm front calipers and six wheel disk brake calipers in the rear, I replaced my P-30 master cylinder with a 34 mm GM. I used a CarQuest 20-2234 (which corresponds to a GM 14009146). It is working very well for me while using the original brace PLACED BETWEEN THE MASTER CYLINDER AND THE BOOSTER. You have to flatten the bolt hole tabs a little to have a good fit against the rear of the master cylinder. It gives plenty of clearance between the end of the master cylinder and the front door. It also has the metal reservoir instead of plastic ones that I have also seen.

The GM 14009146 master cylinder crosses to an AC/Delco 174-834, a Wagner F-103239, Bendix 11641, ElS E150063 and a CarQuest 20-2234. This is a 34 mm piston bore (approx. l-2l/64”). This fits a 1977-96 P35/P3500 Van (with 4 wheel disk brakes) and a P-30 Van. It also fits a 1991 C3500 Pickup. It will give sufficient volume for the use of 80mm front calipers added to the front wheels of the GMC and for 4 calipers added to the rear wheels. It will
fit behind the front cover (below the windshield) without any modifications to the cover or the necessity to shim the hinges.

This master cylinder has a l/2x20 threaded port for the front chamber and a 9/16x18 threaded port for the back chamber. NOTE: the original GMC motorhome master cylinder had the 1/2 in the back and the 9/16 in the front. So it is necessary to move the 9/16" (rear brake line) to the rear chamber and the l/2” (front brake line) to the front chamber. This is not a problem as this master cylinder has two chambers that are of equal size - unlike the original one that has a smaller front chamber that went to the rear brakes.

My write up from 2004 on this site details my use of a P-30 master cylinder with a 1-5/1 6" bore. Ignore that information now as II have now replaced it with this 34mm master cylinder.

I have found that my original master cylinder brace fits just fine BEHIND the master cylinder (between the booster and the master cylinder) so it was not al all necessary for me to fabricate a new brace. To release the rear clip on the cover of the master cylinder just push it with a screwdriver and it will move off the cover to the rear. It can be tightened back on by using a screwdriver to one side to pull it back on. I experimented with the rod length and find that a rod with a length of 2.63" works fine. I used a piece of drill rod the same diameter as the original rod. I ground a spherical shape on the end that goes into the master cylinder and just chamfered the flat end that fits against the booster.

This setup gives me very good pedal and good stopping power.
Emery Stora

How the Master Cylinder Works

Rick Staples
A so-called  "dual" master cylinder (like ours) has one piston attached to the pedal, and  another floating piston further down the same bore.  As you depress the  pedal, the primary piston passes the compensating port (little hole  connecting to the reservoir) and starts to build up pressure.  Fluid is lead  from this chamber of the cylinder via a pipe to one set of brakes.  Pressure  in this primary chamber also presses against the secondary (floating) piston,  and IT moves down the bore, passing ITS compensating port and building  pressure in the secondary chamber, where another pipe leads to the OTHER set  of brakes.

   If either set of brakes (front or rear in our case) springs a leak, the  corresponding chamber empties, and that piston "bottoms out".  If it is the  primary piston, it moves down until it physically touches the secondary  piston, and now pressure is applied directly to the secondary piston.  If it  is the secondary chamber that empties, the secondary piston bottoms at the  end of the cylinder bore, and pressure builds up between the two pistons in  the primary chamber.  In either case, THEORETICALLY  you still have the  remaining half of the brakes.  (In fact, the DOT required manufacturers to  install a brake warning light to remind the poor befuddled driver that a  failure had occurred, since this system worked so well we might not notice  the problem otherwise!)

   HOWEVER, in the real world it doesn't work that well.  Usually the problem  is brakes which are out of adjustment, have a little air trapped in lines or  cylinders, or spongy hoses, etc.  Any/all of these things cause a low pedal.   When one of the sets of brakes fails, you lose about 2/3 of the pedal travel  before its piston "bottoms out".  If the remaining brakes are a bit  low/spongy, then the pedal hits the floor before the remaining brakes fully  apply.  (Your case.)  Sometimes pumping the pedal will bring it up enough to  work, but the GMC is hard to pump effectively.  Also, the GMC brake system  works hard even if it's all there, so any failure is more dramatic than in  your car.

   BTW, the warning light/proportioning valve has little or nothing to do  with all this, except as another place to trap air bubbles, or as as a  potential leak between front and rear systems which might circumvent the  intended separation.

Maintain the OEM brakes with the following: (11/23/06)

Best to use Performance Friction Company's Carbon Metalic pads for best stopping ability & much....much less wear on the Rotors.  Auto Zone Parts house carry our pads (0524) in stocks for ~$26 with a written 999 month warranty...Best Deal in town if you have a Auto Zone Store.Duane

Performance Friction # 524 (0052.10) and #6144 (0614.3) both fit our calipers and the 80 mm.calipersDenny Allen

 Front brakes
        Front Calipers  OEM (not over size)
        Brake Pads      Carbon Metelic (Performance Friction Co)
        Pad P/N         0524  (about $40)
        New Hoses       ????? Long Toronado (2)
BH 36675 is the correct Raybestos number for our coaches. Other numbers are Wagner F86578, Autozone 88498, Carquest or EIS# SP5363. These also fit 70-78 Toronados. If the parts guy asks "which side?" the answer is the right side. The Toronado left hand side ones are to short for our coaches.

Brakeware 77320 & 77421 are the p/n's for the left and right hoses,respectively.  According to AC  Delco reverse lookup, 93 Chevy P30 van (or 586 others) is an application.  You'll need banjo bolts to go with those also. Ken H. (11/23/06)

I called my local AutoZone in Santa Fe and these numbers, Left 77320 and Right 77421 are correct numbers and they have both hoses in stock.  The price here is $24.99 each plus sales tax. Emery Stora 11/23/06)

The pads fit the same year Toronados. I prefer the "Performance Friction" brand of carbon metallic material and their number is 524 with 6144 as a second choice. The 6144 has a little more friction material so should last a little longer.Denny Allen  (8/22/05)

        Rotors can be obtained from Cinnabar for $125 each 1-800-720-2227
Rear Brakes
       Mid Axle Cyl    1 1/16"  Wagner F79767
        Rear Axle Cyl   15/16"  Wagner  F51081 OEM Size
2 sets  Brake Shoes     Asbestos Only  GM #8020290/Delco -- BUY FROM
Gene, and all who may need this info.  I just put new 15/16" Wheel Cylinders on my 3rd axle.

Here are #'s take right off of the box
Dorman  W51081
Which replaces (as noted on the Dorman box)
Coniseal WC13469
Raybestos  WC37048
Wagner  F51081
Bendix  33469   LARRY 10.27.13

GATEWAY-- #171-500

John Evans  he has the carbon met shoes,
he also has front pads if you do not have a Autozone, no cores needed

Try calling him at this # 716-652-6868

        Springs         New Sets
        Combination Valve (Brass)       GM #25509419

The proportional valve is available at GM Dealers under part#1236004.  I just got one last month.
They might have to order it.  As for bleeding I pulled the valve out not in. The real question is that do you have air in the master cylinder and if you do then you will not get any fluid to the rear brakes.JR

In doing my brakes a few years ago I ran into the poor to very poor and bad "STAR" adjusters on the rear brakes.  This coach is made to be self adjusting when you back up. Of course the self-adjusters will not work properly or at all if the "STAR" is not in servicable condition.  chuck