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M35A3 Locked Up Brake Issue - Identifying and Fixing

mactiredearg

New member
199
1
0
Location
Denver, Colorado
Awhile back I had the non-releasing brake issue that some of the A3 owners are running into (http://www.steelsoldiers.com/showthread.php?88172-M109A4-brakes-locked-up!). I finally got it figured out thanks to a lot of suggestions on here (particularly peashooter) and wanted to post some pictures of the issue so other A3ers will know what to look for. I also thought I'd cover part of the master cylinder replacement with some pictures.

My issue was that the brakes were not fully releasing after each press of the brake pedal. At it's worse I could go from normal press to completely solid pedal in about 10-12 steps on the brake pedal. The only way to get the brakes to release was to roll under the truck with a wrench and unscrew the bleeder valve on one of the airpacks enough to drain some fluid.

The cause of this issue was the brake pushrod falling out of the cup in the face of the master cylinder. It then rested on the face of the master cylinder and would not let the master cylinder piston return far enough to expose the return port. So in effect I was pumping the brakes without releasing any pressure until it reached max pressure and the pedal wouldnt move.

Here are:
1.pushrod.JPGa picture of the pushrod

2.pistonface.jpgthe master cylinder piston face (note the marks from where the rod was hitting it while out of place)

3.badposition.jpg a picture of the rod in the truck when it has fallen out of place (see how the adjusting nut is touching the bottom of the boot)

4.goodposition.jpga picture when the rod is in the correct position (see how the adjusting nut is touching the top of the boot and is more level).



The short term (get yourself home) fix is to roll under the truck with a wrench and loosen the airpack bleeder valve and let it dribble until it stops then get towed home. This release will get you maybe 8 or 10 stomps on the brake pedal before you have to do it again.

The long term fix (once home or if you have enough time in the field) is to remove the pushrod and adjust it to make it longer so it will quit falling out of the cup. You will need to test the adjustment to make sure it keeps the pushrod in the cup and doesnt over compensate and cause the same issue by pushing too hard on the piston.

Here are the only pictures I have of removing and reinstalling the pushrod. It's about the easiest piece of work you can do on the pushrod. Just pull the cotter pin, slip the spring off the pin holding the pushrod in place and pull the pushrod out.
5.spring.JPG6.spring.JPG

Note - DO NOT LEAVE THE TRUCK SITTING IN THIS CONDITION. I was _stoopid_ and did exactly that out of frustration. What would have been a simple 5 minute fix turned into a couple hundred dollar repair. The secondary piston in the master cylinder will get STUCK in the fully compressed position if left for too long and you will need to replace the entire master cylinder to get this fixed. Learn from my stupidity.
 

mactiredearg

New member
199
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Location
Denver, Colorado
Now on to the repair caused by my stupidity: A Master Cylinder Replacement. I wont go over everything because the TM (TM9-2320-386-24-1-1) covers it perfectly and I basically followed it step for step. But there was a trick or two picked up from suggestions of folks on here that I'd like to show in pictures for others.

First, the socket setup. Since the master cylinder is so close to the frame it is nearly impossible to get a wrench or socket onto the frame side bolts. The tip I used was to put a deep socket onto a universal joint that is connected to a long extension that is on a universal joint that is connected to another long extension. The pictures below explain it and the routing better than I can. Without this jerry rig there is absolutely no way I could have dealt with the bolts on the frame side of the master cylinder.
1.socket.JPG2.socket.JPG3.socket.JPG



Second, even with the socket thingy if you have big hands like me getting the frame side bolts started is an exercise in perpetual frustration and a really good way to get them cross threaded.
4.hands.JPG

So get a Soldier B with small hands to help:
5.soldierb.JPG6.soldierb.JPG

Just make sure she has her eye protection on before she dives right in and starts working:
7.soldierB.JPG
 
Last edited:

peashooter

Well-known member
1,038
205
63
Location
Hanover, minnesota
I have a couple master cylinders available for these, including 1 less expensive type that has a lead in rather than the dimple hole on the piston. Now I can see why that would help with the problem some of us have ran into! Thanks for taking the time to write this up w/pictures!
 

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boudreauxs ride

New member
3
0
0
Location
slidell,la
My brakes were locking up too, after a close equation I decided to remove the master cylinder and found the inside had rusted {this truck was used for high water rescue by the local fire department} Long swivel a u shaped 9/16 wrench and a lot of patient's was able to do the whole job in apox 3-4 hrs. I did find a master cylinder at my local O'riellys auto parts. My commercial guy ordered 4 units before we found one with the same ports, had to remove the reservoir remote top on old one and installed on the remand one. Total cost $69.00 Lot better than some of the other places I searched $1200.00 to $450.00. I don't have the part number on me right now but if anyone would like it i'll post it later
 

61sleepercab

New member
622
3
0
Location
Walton, West Virginia
The master cylinder push rod and the clevis bolt are used to adjust brake pedal free play. You can adjust the pedal to take up excessive free travel, but you have to make sure that the master cylinder rod does not stop the piston from returning to the stop position which allows the compressed brake fluid to release through the vent hole and bleed back to the reservoir.
I had a International truck that had touchy brakes and I had to adjust rod to gain pedal but not to far or truck brakes would be set and not release. Mark
 
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