Thanks to
Ken Henderson
 Gary Zingle
Mark Wall

1/2-20 & 9/16-18
2.469 in


31.75 mm
1/2-20 & 9/16-18 HYD ASSIST DIESEL

lll.258 108145

33.337 mm
9/16-18 & 1/2-20

3.6875 in

F103239 MC39309
1.338 34 mm 9/16-18 & 1/2-20 14009146 39-622 F103239

* Wagner MC numbers  start with F

Harrison's 34 mm Master Cylinder

Yes, I talked with Leigh Harrison.  The master cylinder he uses, one that fits the GMC's stock vacuum booster, is GM #14009146.  It is for a P30 cabover chassis and readily available, but expensive.  It costs $302 in Seattle.  For me, it's worth it.  It fits in the available space, and I don't have to shim the door out.

One thing Leigh said that might be of interest to others, concerns the longevity of the master cylinder.  Since there are no seals on the front access doors, water  pours in while driving in the rain.  He suggests wrapping the master cylinder in aluminum foil as an extra barrier against water.  He said water is the brake systems greatest enemy. John

Here is a picture of tne NON- rectangular P-30 Master cyl.   It is a little too long

Having given the above info, a larger master cylinder (from the Chevy P-30 chassis) and adjustable brake rod (goes in the chamber between the master cylinder and the brake booster) (or a rod cut to length), increased, I believe, 0.010 " greater than the original length, is required.

Master cylinder (34 mm): 14009146 39-622 F103239

A master cylinder mounting bracket needs to be fabricated in addition. Help on that requirement is available upon request.

Paul Bartz

First of all, my thanks to everyone who contributed to helping me get my brake system upgraded and functional. My special thanks to Ken H. and his wonderful brake pedal pusher. Without that thing, I wouldn't have been able to try all the different master cylinder combinations because my wife would never have stood still for the hours required to bleed the system out to ensure even test results. Here's what I have now:

I am able to lock up the brakes pushing hard with two feet after the pads warm up during normal driving. Still don't know which ones are locking first because I had no one to observe and I didn't want to flat spot the tires. Next step is to have someone watch and see what is happening. Hate to throw away $1,000 worth of tires to prove out the system but hopefully I can avoid that and still find out. 

Locking up the wheels. What I am really after is getting the brakes clamping to the point where they are just about to lock - optimum braking. Unfortunately, the only way I know to find that out is to try to lock the brakes. From that point, I can, in theory, modulate the pressure to ensure maximum braking effectiveness. Since it takes some serious leg pressure to lock them with the current P-30 master cylinder, in theory, I should be able to maintain unlocked, but maximized, brakes under all conditions. Even, in theory, in a panic stop. Note the 'In theory' part.

Master Cylinder that worked for me with the TSM brakes was a P-30 from a 1977-1992 with 4 wheel disk brakes. I asked for one from a 1990 but the Raybestos website shows the same part number for the years I listed.
MC39309 Rabestos #,
39309 Napa new #,
10-1668 Napa reman #. $53 with a $56 core charge.

This unit has a large rectangular reservoir that is made of cast aluminum and is bolted on to the cast iron cylinder.

For the record I am running the Harrison sensitized booster, an MC from a 4 wheel disk 1977-1992 Chevy P-30, TSM disk brakes, 80mm fronts, Dot 4 fluid, Performance friction pads on all six wheels, Alcoas and Michelins.

Master Cylinder Mods. I did have to slightly modify the mounting bracket to fit. The bracket would not clear the cast aluminum body so I had to grind off part of the brace that is welded to the support bracket and I had to jog the lower ears forward a bit. A little flaming wrench and die grinder and about 15 minutes of work. I still mounted the parts in order of booster/MC/bracket. The other change was that the brake lines DO get reversed for this P-30; the front brake lines are now FARTHEST away from the booster. The stock line fittings work fine, just swap the order of the lines. The only unusual thing about the application is that I have to remove the rear bail completely off the MC in order to get the top off. It hits on the bracket and doesn't clear the MC cap.

Numbers. Using the Waekon gauge, I was able to measure the clamping pressure of the calipers. Using two feet, I got the following results for the rear calipers with the various master cylinders I tested:

1976 Corvette with a 1 1/8 bore: 1,000 pounds
Stock GMC with a 1 1/4 bore 3,200 pounds
P-30 with a 1 5/16 bore (972-1976 with 15" drums 3,300 pounds (also was so long that interfered with the hatch closing. Left it open by several inches)

P-30 with a 34 mm bore(between 1 5/16 and 1 3/8) (I am trying to verify the 34mm measurement without removing the damn thing and measuring it. It may only be 1 5/16) 4,000 pounds

The rear caliper has the smallest piston area of all my calipers. Using the Waekon results and the piston area, I calculated the line pressure to be approximately 1273 psi under maxium braking pressure using two feet.. Using the piston area for the center and front caliper pistons, I calculated the clamping pressure to be approximately 5,600 pounds for the center calipers and approximately 9,000 pounds for the 80mm front calipers. Unfortunately, the Waekon gauge only goes up to 5,000 pounds and the center and front brakes exceeded the gauges capacity. I am confident of the center numbers because they are on the same circuit as the rears - same line diameter, same MC piston, etc. The fronts may or may not be accurate. I have no reason to believe that they would be wrong, just no way to ascertain that they are absolutely correct without measuring the line pressure under maximum braking effort which I did not do. During testing the line pressures were pretty consistant at all six calipers at the same brake pedal pressure.

Normal braking is great (subjective) once the pads are warm and I am exceedinly happy with what I have right now. The only thing I am still curious about is whether all SIX wheels will actually lock. Maybe I can try it on wet pavement without doing too much damage to the tires or maybe I'll have to wait until I am ready to replace this set. I can tell you this though, hearing the beginnings of a tire screech during the brake test made my day.

If anyone has any questions about the numbers, or if they seem suspect, let me know. I was using a line pressure gauge during the tests but, unlike the Waekon gauge which has a dynamic needle and a second needle which indicates maxium results, the line gauge only has a single dynamic needle and I couldn't get my wife to come out and play during the two foot panic tests.

For the record I am running the Harrison sensitized booster, an MC from a 4 wheel disk 1977-1992 Chevy P-30, TSM disk brakes, 80mm fronts, Dot 4 fluid, Performance friction pads on all six wheels, Alcoas and Michelins.

Again, thanks for all the help and I hope this information is helpful to everyone.  Mark Wall 76 E2