M923a2 air question

Hi guys, I searched and read the tm and I am still at a loss. When the m923a2 is running at a stop with the air pressure up and I step on the brake peddle I loose 20 psi air and it vents out the intake stack. Is this normal or is it an abs problem? Thanks Joe.

Its the q2 valve under front bumber just in front of engine.

Coffey1 said:

Its the q2 valve under front bumber just in front of engine.

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Is it a problem with that valve or normal operation?

Must be a problem. Do that a few times in a row and you will have no air. And I don't mean A.C.

When you RELEASE the pedal, you will lose air, yes.

If you're just pressing down and holding steady, the needle should settle and not leak down more than 1 psi per minute.

If you have a leak down problem while firmly holding the pedal down (steadily), have someone go underneath the truck to see where the air is leaking (listen at each cyl)

Do a search for COLA air brake test.

• C = Air compressor cuts in.
• O = Air compressor cuts out.
• L = Low pressure warning.
• A = Air leakage.

Thanks

Its that q2 valve when you hit brakes and that valve bad it will dump all the air into the vent stack.
There's what looks like a nylon ball in that valve that breaks and will not hold air when you hit brake pedal.

Question, M923 A0 no CTIS and Super singles.

I have this same type of issue, but you can hear air coming our from around the wheels on the front left and rear most right. I do hear the air going through the vent stack. We checked the lines and everything under side with soapy water and no bubbles when on and off the brakes. It sounds like air is leaking into the wheels brake drums.

What I am thinking is that maybe a diaphragm is bad in the canister.
Would that case air to leak when on the brakes in the wheel area and vent stack? (Only when on the brakes).

My mechanic is going to follow the steps in the TM to check this, but looking for real world advise from the pros. I know it is VERY dangerous to change the diaphragms out and will change the whole part if needed.

Thanks in advance.

K9 there is no danger in the front to change diaphragms.
Only on the ones with spring brake. Those are the ones with holes in the ends of the canister.
When the q2 valve is bad you will dump all air into vent stack every time you hit the brake peddle.

Coffey1 said:

K9 there is no danger in the front to change diaphragms.
Only on the ones with spring brake. Those are the ones with holes in the ends of the canister.
When the q2 valve is bad you will dump all air into vent stack every time you hit the brake peddle.

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If you decide to dissemble the rear I strongly suggest you don't.

Had a section sgt have me do it and if it wasn't for dumb luck I wouldn't be here today.

Thank you all and trust me, I WILL NOT make any attempt to change the diaphragm on the rear. I even told my mechanic to not do it or even think about taking them apart. He is a friend that owns a shop and is helping me out.

I will check that front q2 valve and see if that is it.

I looked through the TMs and see the TS guilds I do see my issue exactly and does say it may be the brake chamber. However, I did not see the q2 valve mentioned. Can someone post a picture or a part number for that?

Thanks.

Coffey1 said:

K9 there is no danger in the front to change diaphragms.
Only on the ones with spring brake. Those are the ones with holes in the ends of the canister.
When the q2 valve is bad you will dump all air into vent stack every time you hit the brake peddle.

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Is there anything special to pay attention to when changing the diaphragms on the smaller canisters?
I want to just replace them all since it is safe to do these on the smaller canisters.

Part# AD17544
Found them for $6.25 from surplus parts store. Per my old school math, there are 6 of these total; correct?

EDIT: What is the NAPA part number, is it the same MBI DP12?

Coffey1 said:

K9 there is no danger in the front to change diaphragms.
Only on the ones with spring brake. Those are the ones with holes in the ends of the canister.
When the q2 valve is bad you will dump all air into vent stack every time you hit the brake peddle.

Click to expand...

What do you mean by "dump all air"? Does that mean if I hold the brakes, air will continually vent into the stack? Should not some air vent into the stack when you hit the brakes?

It will dump all air as long as foot is on brake peddle.
Q2 valve is dead center front. Hanging in front of engine.
Can't miss it. It looks like it should not be there

MBI DP12 is the Midland part number. Just ask for size 12 brake diaphragms. There are several acceptable manufacturers and they are all subject to the same DOT specs.

Coffey1 said:

It will dump all air as long as foot is on brake peddle.
Q2 valve is dead center front. Hanging in front of engine.
Can't miss it. It looks like it should not be there

Click to expand...

Well, that's good. I just drop some psi when I press the brake, then when I release, it vents out the stack. no continuous loss.

You will also have continuous air loss while holding the brake pedal down if you have the trailer service line valve open. Don't ask how I discovered that. However, that air will vent at the gladhand, not the intake stack.

Along with the leaking air when on the brakes the primary tank would slowly leak down after turning the engine off to zero where the secondary was still holding pressure. Replaced one of the front brake canisters with a new one and now the pressure holds in the primary tank. Still have a leak through the stack when on the brakes, but it is much slower now. This was only replacing one canister and still have others to replace along with on known bad large spring brake unit. Also gonna replace the Q2 valve in the front near the engine as it was only $20.

Hope to know more tomorrow after the seals are replaced on the none spring brake canisters and the value replaced. The spring brake part will not be in until Monday, but hoping it will all be good without having to replace any of those.

Just wanted to update this post, looking like my mechanic is getting closer to a solution on these brakes. Last is a small fuel leak that should be easy fix. I could do these myself, but the guy buying the truck from me wants my mechanic to do the work.

This is from one of the small canisters, looks like that pinch may have been a leak.



This is from the shaft to the spring brake (not the larger 16 in the spring brake).

Pretty much all the diaphragms were in some type of condition that was not so bueno. My mechanic replaced all of them less the ones inside the spring brakes and now the system holds air when on the brakes. The pressure builds evenly and much quicker. It holds the pressure in the tank when the truck is off sitting as well. Still needs some long term testing, but everything is looking good now. I did not need a new small canister, that was my mistake not knowing those can be rebuilt safely.

So I spent an extra $90 on that, but at least you can see brake work was done. I have one new spring brake canister as well that was not needed. We though one was bad, but turned out okay, so that will be a spare if ever needed.

I guess this truck either sat for a long time and they started to dry out, or they were replaced poorly by the military at one point. All of them pretty much has that crimp in them like the images above. Small cracks and some of the metal canisters had corrosion build up.