M35A2C master cylinder failure?

[peashooter]

First, make sure that after pumping the pedal, when fully released, the (primary)piston is returning all the way back to its stop. Then, see if you can feel some play between piston and pushrod, even with the brakes/wheels locked.


G.

Gringeltaube, you nailed it again! After much frustration and work (not to mention Money spent ordering some airpack rebuild kits and orings for the MC) I checked the pushrod, and found that it had come out of the piston bore in the M/C so it was pushing against the face of it. So as you suspected, the compensating valves would have been blocked. My wife and I thank you (I was putting way too much time into finding the problem).
I guess I have a jumpstart on the parts for next years brake overhaul anyway.

 

[gringeltaube]

Glad you found the REAL problem!

Now that we know it..., wait a minute, this is NOT the first time someone on here had exactly the same problem! Which also means, it can (and according to Mr.Murphy - will...) happen again!

If I could find that thread, now...


G.

 

[m-35tom]

it really scares me how many people who really don't understand what they are doing, are working on brakes. brakes are pretty important you know?

 

[oifvet]

peashooter,

I thought about that, but I was afraid that the diameter of that vent line was just too small. I suppose pressure is pressure, but if you have to move fluid, it might slow the process down. It wouldn't hurt to try.

Also, my issue with the fading pedal was due to me installing the two little spring-loaded check valves backwards/up-side down. Once I paid better attention to the schematic and flipped them over, all worked perfectly. Removing the MC went quicker the second time. The re-bleed worked much easier, too. I did not need a pedal-pumper. With the valves installed correctly, the new fluid pushed through easily and the entire bleed process on the dual-circuit took place in no time and without one cuss word! I have brakes!

 

[peashooter]

It is the same MC on the A3's.

Just for future reference for people with the dual circuit brakes on their 87-89 M35a2/M109a3 trucks and M35a3/M109a4 trucks, the Master cylinders are NOT the same. Besides the obvious difference of the A3 trucks having a remote mounted reservoir, they have some other differences.

87-89 A2 Master Cylinders have: built in residual pressure valves (to keep drum shoes close to the drum and not "suck" in air), also they are an uneven split so more fluid goes to the rear 4 wheels than the front 2. Reservoir is mounted to the top of the M/C

93 & newer A3 Master cylinders have: NO residual pressure valves, and have a 50/50 split so a single brake pump puts out the same amount of fluid to the rear 4 wheels as it does to the front 2 (before the airpak kicks in). Reservoir is remote mounted

It appears to me currently that the better designed system was the late model A2 trucks. I'm replacing my 88 M109a3 Master cylinder but just adding the remote reservoir mounting plates to it in order to get the best of both systems.

Im just not sure if the airpaks from the a3 trucks have Residual check valves or if they just chose to not have them on the system at all.

 

[SP5]

I know that this thread is ~3 months old, but I thought I might add a bit to it. Since coming-on-board, I've been reading thru previous posts, going back,(so far), ~220 pages, taking/making note of everything concerning M35A2 brakes.
It seems that some of the wheel cyclinder rebuild kits have cup expanders in them, and some don't.
Without a cup expander, it seems that a residual valve of ~10lbs. would go a long way towards preventing the "leak down" of the brake system.
A residual valve does not keep the shoes closer to the drum, the return springs are so powerfull that they will always retract the shoes back as far as the adjustment cams will allow.
But, a residual valve will hold a little pressure, so as the keep the cups expanded and prevent leakage.
Now, in many older vehicles, the residual was installed in the return port of the MC, in a deuce, with an airpack, perhaps a better location would be in the main line downstream of the airpack??
It does seem that a combination of cup expanders and a 5>10lb. residual would surely help in retaining fluid in the system.

 

[peashooter]

Im just not sure if the airpaks from the a3 trucks have Residual check valves or if they just chose to not have them on the system at all.

I also learned that neither airpack (longer old style, nor the shorter new style) have residual check valves in them.... although you can get kits for them that install on/in the end of the airpack which would be a good spot to put them as SP5 mentioned.