|THIRD BRAKE LIGHT
|SUMMARY CONFIGURATIONS||OEM BRAKES REBUILD||PRESSURE BLEEDER||BUILDING A BLEEDER|
|80 mm UPGRADE PARTS||DOING THE 80mm UPGRADE||BRAKE ERROR LIGHT FUNCTION||Stainless Brake Lines|
||ENHANCED BOOSTER||DISK MASTER CYL||SINKING PEDDLE|
|Disk Brake Differences||Cadilliac Rear Disk Brakes||PARKING BRAKE PULLEYS'||Brake Analysis|
BRAKE SYSTEM CALCULATIONS
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
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 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
|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
that have done the cad rear brake conversion do not use the cad
with the emergency brake feature. It has a tendency not to
release and the rear brake stays locked. The base set up is the
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
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
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
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
Mouse skinny spares) for the air source. This has the advantage
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
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
BUILDING A GMC BLEEDER
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 project.....my old hoses came off easy....lucky me!! Larry :-)
80 MM Front Brake Caliper Upgrade
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
of months back, you may want to consider going to a larger master
if making the switch to larger the calipers. If so, it's
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
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
it is truly a brake warning light and should not be
It is triggered by the proportional valve located on the left
behind and above the driver's side front wheel. The
valve has brake lines going into it from the master cylinder and
out to the front and rear calipers and wheel cylinders.
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
The same thing happens when the back brake circuit has lower
Marlene -- try pulling the wire clip off the top of the
valve (turn on your key) and with it disconnected the brake light
should go out. When you ground it to the frame or other
it should light. It may be that you've got a short in the
somewhere or you might actually have a brake problem. A
wheel cylinder could cause this. A faulty proportional valve
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
in the front brake circuit and trigger the light. Bleed the
master cylinder and then all the wheel lines.
Stainless Steel Brake Lines
Disk Brake Differences
- 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
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
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
My solution was to put on a larger master cylinder as shown on my
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
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
by looking at them without disassembling anything, very easy to quickly
replace the pads when needed, no need to periodically adjust the rear
and no fade or vapor lock due to the cooling running inherent in the
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.
J. R. Wright
Combination Valve (Brass) GM #25509419
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
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--
DISK BRAKE RETROFIT
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
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.
McMaster-Carr (http://www.mcmaster.com/) pulleys.
P/N 3099T52 and they sell for $5.36 Total shipping was $2.17.
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
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
http://members.aol.com/adohen1/page/Adohensupply.htm Ahohen Supply Co.
2) The OEM installation of the brake cable had a hook rod on
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.
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.
HORROR STORIES OF LOST BRAKES
If your parts supplier can not get a Wagner, then he can cross
to local brand. Don't get stuck with a rebuilt one, not worth
home & small difference if bought Right. This is a perfect
for all '73 thru '78. It's best to bleed the master cylinder on the
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
Power of the "Net" illustrated again !!!!. Duane Simmons
WHAT MASTER CYLINDER TO USE FOR DISK BRAKES
How the Master Cylinder Works
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
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
Performance Friction # 524 (0052.10) and #6144 (0614.3) both fit our calipers and the 80 mm.calipersDenny Allen
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.
Rotors can be obtained
Cinnabar for $125 each 1-800-720-2227
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
Which replaces (as noted on the Dorman box)
Bendix 33469 LARRY 10.27.13
John Evans email@example.com
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
Combination Valve (Brass) GM #25509419
The proportional valve is available at GM Dealers under
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
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