M923 air pressure problem

More information would be needed. Did you look for leaks? How slowly do you build pressure does it come up to 120psi? Was it working before or did you get it this way? Do you build enough pressure to release the brakes?

So, the truck does hit 120 psi at idle, takes roughly 15 minutes. It's been like this from the start. Bleads out in about 30 minutes back to zero I did the old soap and water trick on the entire truck. I hear some air passing through the air brake but not leaking out of it. I hear air coming from the intake stack. That is what perplexes me. Why are there 2 air lines going into the stack? Can I just plug the things? Probably not. I'll bet it's a safety so that something else doesn't fail.

Perhaps pump the system from a shop compressor so you are not combatting a noisey running engine... I added a Schrader valve via a 1/2" brass tee at a PRV I have at and above the frontmost rear axle, DS, it is before the desicant air dryer. Have you tried shutting down the primary and or secondary system via the onboard valves in an attempt at trying to isolate?

I can hear it leak from the air intake from when the engine is shut off until the tanks are empty. No doubt that's it. I am not sure about isolating it. That sounds like a good start.

The TM lists my probem word for word. I just can't figure out their troubleshooting guide. Air leaking into the intake can only be several things.

Perhaps a check valve... No promises but give me the exact location in what TM for the WROD FOR WORD description you speak of...

Trace the airline back that is venting into the stack and you will more than likely find your problem. Might be a ruptured diaphragm in one of the air brake chambers, don't take the chambers apart if you don't know what you're doing, there is a strong spring in there that can cause injury.

will have to wait until the weekend. thanks for the word of caution. I hear those are some heavy duty springs.

Any updates on this ? I just picked up my m923A1 today. It takes forever to build air . If you throttle up to 1500 rpm it in about 6 min will get to 125 psi . But at idle it takes forever to reach 90 psi. If left sitting at idle the secondary pressure will hold but the primary wont. If I shut the engine off there are no air leaks to be heard ?? going down the road it will maintain 120 psi ??

Sticking unloader or leaky air dryer most likely

Yes. Pressure up the system and shut the truck down. All leaks should be sent to the hoses going into your intake pipe. Follow the air back from there. My leak came from the foot valve. Everything worked fine, but air went to the brake pedal and was able to billed off. Also, these trucks build pressure rather slowly. Good luck

I have a M925A1 as well and its at 120 in 3 min or less. the m923A1 was at 6 min to make a bit more than 60 psi. I found another post in my search that sounds closer to my problem . It involves a stuck check valve. I'll look into that as well as other options. If the truck sits for a hr off it really doesn't loose much air Maybe 10 psi from what I saw. so there may not be a leak ?

R Racing said:

I have a M925A1 as well and its at 120 in 3 min or less. the m923A1 was at 6 min to make a bit more than 60 psi. I found another post in my search that sounds closer to my problem . It involves a stuck check valve. I'll look into that as well as other options. If the truck sits for a hr off it really doesn't loose much air Maybe 10 psi from what I saw. so there may not be a leak ?

Click to expand...

There are plenty of hoses and valves on those trucks. One thing that helped me was to isolate the air tanks. There should be shut off valves on both sides. It helps to rule out one system at a time. Even if it's not a leak.

Good luck.

Reviving this thread.

Working on a problem that just started. Going down the road and hit the brakes and my air pressure drops about 25 to 30psi on the emergency tank. Secondary doesn't move. Can hear it coming back out of the intake tube when I pump the brakes when I am sitting still. I do not hear it when I am driving and apply the brakes. Pedal to the floor, hardly wants to stop. will but pedal becomes non existent to stop it.

Closed off the valve on the front of the first tank. hit the brakes and hold em and it falls to from 120 to 0 in about 15seconds. Left that one closed and closed the valve on the top of the same tank. Same result. Opened the front valve and kept the upper closed and still the same result.

Caged all the rear brakes at once. Same result with pressure falling. No leaks anywhere. I used shop air to check the system. Also checked all gladhands they are closed and not bleeding off thru them. It does act like the service port is open but the one on the rear is not open. I do not hear anything from the drivers front with shop air applied and brakes applied.

Just installed a new MRAP dryer a week ago. Did not have this problem until last night. Going to check between the vent line and the line going to it. along with around the pedal I guess. It seems that maybe a check valve is stuck but dang a lot of air to just go somewhere and not hear it.

Does the treadle valve feel spongy or different than normal. M939 treadle should be really firm.

It is firm for the first few seconds of the pump then it fades off and when it hit bottom I have no pressure on it and It is spongy then.

I was thinking the treadle cause when my emergency air gets low 60psi or so my secondary tank does not lose an air at all at 120+

Unless your brakes are caged there should never be a situation where you step on the brakes and the truck does not
want to stop. The treadle should be firm with or without air in the system. Spongy is bad.

Yea at the moment it is slow. I mean I don't use my brakes much anyways. Letting the downshift bring me to a stop almost. Especially in traffic keeping distance. And I am never in a hurry so. I don't tend to use them very hard but yeah here as of today pedal to the floor and it slowly stops stops. It is like a regular passenger car brake system with air in the rear wheel cylinders. I will swap in a pedal off a parts truck tomorrow and see what happens.

To be honest It almost feels like only the fronts are working. I don't hear any squeaking like I normally do from the rear without them caged.

Rears are spring brakes air keeps them off the treadle takes the air away when pressed. So they really can't just not work. Unless air is somehow leaking thru the treadle valve and the flow does not stop when you step on the pedal. Chances of mechanical brake failure of 4 separate spring brakes are pretty unlikely.