Deuce brake problem

[wildbill81]

The brakes on my 1967 M109A stay applied after a couple pumps on the brake pedal(hard pedal, brake lights on, truck no-move). I've read many threads about the possible causes of this and have tried all the solutions to no avail. I can get the brakes to release by opening the drains on the air tanks. I ordered a rebuild kit for the air pack, but unfortunately it was for the long and mine is short. Regardless I disassembled the air pack and it was extremely clean and everything had fresh grease as is if it was recently rebuilt. All parts moved freely and all seals were in excellent shape. I rebuilt the master cylinder and verified all passages were clear. After reassembly, the same problem persists, just takes a few more pumps for the brakes to stay applied. With the engine off, I can pump the brakes all day and hear the air exhaust off the air pack when the brakes are released(at least until the pressure drops too low) so I know at least some air is being ported off the air pack upon release of the brakes. The only modification to the system I have made was to plug the service glad hand connection to the rear when I bobbed the truck. Am I missing something here? What is the normal air system pressure, could I have a bad/improperly set unloader on the compressor?
Thanks

 

[gimpyrobb]

Yup, I have seen glad hand covers with vents on them. Might want to try un-plugging that line.

 

[wildbill81]

I drilled an 1/8in hole in the plug and the brakes work much better however if I hold them on for a minute or 2, the air pressure goes down and I can feel the 'assist' fade away. I can at least drive it around the yard I may try a new plug with a smaller hole since I don't plan on re-installing the service brakes anytime soon.
Thanks

 

[doghead]

By drilling the hole in your plug ,you are relieving the air pressure that is leaking by in your air-pack.

Fix the problem.

 

[gimpyrobb]

Sounds like there is still an issue with the airpack. You *should* be able to have that line plugged and not have the brakes lock. With the hole in the plug, the airpack is sending air out that line trying to control a trailer. It WILL empty your air reserve quick if you are not careful. Maybe someone more familiar with the airpack operation will help us out here.


Edit, DH beat me to it.

 

[doghead]

He could also have a mis-adjusted brake pedal to master cylinder push rod. A sticking M/C, or most likely a bad air valve on the air pack.

I would rebuild both and properly adjust the push rod.

 

[peashooter]

I had the brake pedal to M/C pushrod problem mentioned by doghead. In my case, the pushrod came out of the "pocket" in the M/C piston and wouldnt let the piston fully return to its home position which prevented some internal valve from releasing pressure. It ended up being a easy fix, but took days and lots of money replacing parts and re-bleeding each time before Gringletaube magically figured out my issue from across the globe.
I find if I pull the brake pedal up for some-reason with my hand or foot, it will pop that pushrod back out of the pocket and lock the brakes after a couple pumps. However, the only way to release the brakes was to crack open a fluid bleeder. So I'm voting for the airpack as the problem if relieving air pressure unlocks the brakes.

 

[welldigger]

I keep the valve on my service glad hand closed along with the cover on the glad hand. Something is sticking in the air control valve of your air pack. When you release the brake pedal all air pressure should be vented from the air pack.

 

[rosco]

Its the Air Pack. Your Service glad hand, has nothing to do with it. If you were pulling a trailer, things would even be more complex & difficult to diagnoise.

 

[doghead]

It could be as simple as a plugged hole at the bottom of the m/c.

Find the problem.

 

[leo4x4]

Its most likely the air pack I have an m35a2 had the same issue replace the airpack solve the problem

 

[Adrian A]

I agree with doghead. There are two tiny holes visible inside the master cylinder resevoir. One is a pressure relief. It gets clogged easily. I had this exact issue, anf had to do it twice because i didn't see it. Cheap and easy fix. Try this first!

 

[wildbill81]

Thankk for all the suggestions, I verified both holes in the MC clear when I put the rebuild kit in it, and even with the service port plugged air exhausts from the exhaust port on the airpack when I release the brakes, But I guess it's either not enough, or something is causing air to leak into the airpack and build up causing the bakes to apply. I'll track down a 'short' style rebuild kit and start from scratch...

 

[197thhhc]

The airpack can be venting when you let off but it could still be leaking internally. Rebuilding may fix it, but I lean more towards replacing them. Whenever we rebuild a truck it gets a new master cylinder and airpack. Why take chances.

 

[brianp454]

Is there any reason to believe that a proper rebuild is not just as good as NEW?

 

[leo4x4]

I personally dont like the rebuild kit I know its cheaper. But after all its brakes I just replaced my air pack it solved my problem but now again I have a bad brake Cylinder that wont release. These truck are money pits sometimes.

 

[197thhhc]

Is there any reason to believe that a proper rebuild is not just as good as NEW?

The answer is in your quote. There is nothing wrong with a "proper" rebuild. Proper means honing and measuring the cylinders. Making sure that it is right Not just putting a kit in. If a person has the ability and the tools to do this correctly then it a good and less expensive way to get your truck going.

 

[caliber1]

I rebuilt my Air pack first, but it was past tolerances I guess and still did the same thing. It would suck the fluid out of the MC though. Replacing it fixed everything.

 

[Adrian A]

I rebuilt my entire brake system. Works like a champ. Ive done two air packs and they are good as new. I try to fix everything first. Just so I at least know how it works.

 

[michigandon]

Just started having the same problem, more or less.

Truck in question has a NEW master cylinder and airpack in place.