M35A2 Brake Problem

[ODRotorhead]

M35A2 - Kaiser - 1967 - Unmodified

I'm having problems with my brakes and I'm hoping someone can offer a possible solution.

My brake pedal goes to the floor on the first press. Releasing and re-pressing increases pedal resistance - travel is about halfway to the floor. Third pump - less travel. Fourth pump - the brake pedal is firm and doesn't "bleed down."
Unfortunately, when I take my foot off the brakes for a few seconds, the cycle begins again - First press, pedal to the floor. Second pump . . .

I can't find any reference to that kind of problem in the Troubleshooting section of the TM.
Looked through earlier posts on brakes and haven't found a solution.

Maintenance preformed so far:
Wheels - All six wheel cylinders replaced with new.
Flex lines replaced with new.
Brake shoes are good.
All brake hardware (springs, cams, etc.) in good condition.
Brakes adjusted.
Master Cylinder - Replaced with new.
Short Air-Pak - Rebuilt and reinstalled.
Entire system bled twice - DOT 5 - no leaks anywhere.

My tests were performed with engine running and good air pressure, but only in the chocks. I can't take the truck out on the road until I'm confident I can maintain good brake pressure.

I'm at a loss, so any advice will be much appreciated.

Thanks

 

[NDT]

You still have air in the system.

 

[Bill Nutting]

Bump

 

[Barrman]

How did you adjust the brakes? You should have a firm pedal that is 2-4 inches above the floor with no air in the airpack/truck air system. You wrote it has been bled twice and the brakes are adjusted. Also, that there are no leaks. At least 1 of those 3 still needs attention. Here is a video describing how to do a major and minor shoe adjustment:

 

[ODRotorhead]

Thanks for the tips.

I eventually found a post and a reply from back in 2005 over on G503 that described my situation almost exactly:

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G503 Military Vehicle Message Forums - Login

M35A2s and air over hyd. brakes

Post by Jetnoise400 » Tue Jul 05, 2005 7:42 pm

Greetings one and all,

I have a question for you experienced M35A2 types: What are the symptoms of a brake system air pack (air cylinder, air booster, etc.) failure?

I've adjusted my brakes, replaced the master cylinder, replaced a couple leaky wheel cylinders, and repeatedly power bled the brake system, but I still only seem to get a "low-ish" pedal at the start (1/2 or slightly lower in the stroke), but with 2-4 pumps, I get a strong, high pedal that goes back down when I apply the brakes for the next stop sign. It still stops just fine empty, but I'd be reluctant to drive it this way with much of a load.

I have no detectable leaks, and the fluid level stays constant.

I've been playing this game for way too long, and have run out of ideas. Any thoughts?

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Reply:

Brakes

Post by Rich 24149 » Tue Jul 05, 2005 10:52 pm
I had something very similar on my M35. DOT 5 was burning a hole in my pocket. Assuming you're seeing no air at the bleeders, somewhere there is a shoe(s) out of adjustment. When you pump it, its working its way out to contact the drum (its a distance further than the shoe travels during a normal application) hence the solid pedal. When you let your foot off of the brake the spring pulls it back (beyond the extent of movement possible with one press of the pedal) hence the low pedal. I worked on mine quite some time before I got the idea of removing one drum and seeing how far the adjuster moves the shoe. I discovered the adjustment cam will turn one revolution before it starts over or "cams over" (popping noise and you can not back the adjuster off). Once I discovered this I painted a line on the adjuster at the point of "cam over" and gave every single adjuster 3/4 (more might be needed for worn linings) of a turn. That fixed the problem. Theoretically all the shoes are now the same distance- more or less- from the drum and application should be even. I of course checked every wheel to make sure the brake was not dragging.

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I'm just repeating what I read earlier today. I don't want to put out bad information, so someone please confirm or debunk this . . .

In a different post on a different site, someone said that the short pack was originally designed for the A3 which has two packs. When a single "short" Air-Pak is installed in an A2, it doesn't put out as much fluid volume as the original "long" Pak. The result being, that an A2 with a single short pak is somewhat more prone to brake issues if the brakes are a little out of adjustment or the pads are worn.

True or False???

Anyway, . . . When I last adjusted my brakes someone recommended that I set them at .017 (vs. .015) on the tops and .012 (vs. .010) on the bottoms, so I did. Probably a mistake.

With the information provided by "Rich" back in 2005, I decided to start with major adjustments to the two front brakes just to see if anything might change. After those two adjustments, the first application of the brakes produced a hard pedal about halfway to the floor. A couple of pumps produced a higher pedal, so I have more work to do, but at least I know that the new master cylinder and the rebuilt air-pak are working as they should.

I didn't have time to do it today, but I'm going to do the major adjustment procedure to the other four brakes. I'll let you know the result.

Thanks again.

 

[WillWagner]

You also need to make sure there is free play in pedal. This lets the MC piston retract all the way and release pressure. If no free play, the brakes will lock during a drive.