Bigboy down again,M923 won't build air past 59psi

[tbar123]

I was really excited to get out and drive bigboy for a few miles today since I finally fixed the air horn problem. Everything was great! Wrong!!!!! I started him up and what do ya know he won't build air past 59 psi. So I shut him down and heard air coming out of the intake stack.(video) This is a 84' M923 with the nhc250 engine. Any and all help is greatly appreciated. Have a good one.
https://www.youtube.com/watch?v=olLNmkIO-g4&feature=youtu.be

 

[Suprman]

Is air leaking out the bottom of the air dryer?

 

[63foxtrot]

give throttle to build more air, then thump repeatedly on the brake pedal.

 

[tbar123]

Air is not leaking from the air dryer. I also throttled it up to as high as I want to go and no build past 59 psi. I do thank both of you gents.

 

[Andyrv6av8r]

Unloader issue if coming out of the intake?

 

[tbar123]

Unloader sounds expensive! Is that the exact part description? So I can check on replacement parts. Thanks again.

 

[Plane Fast]

Unloader will not cause air to leak into your intake. The treadle valve and I believe the transfer case vent into the intake. When you push down on the brake pedal, and release. The "goose sound" is coming from your intake. The treadle valve maybe stuck open.

 

[tbar123]

Can that be rebuilt?

 

[Plane Fast]

Im sure it could. ( I do have two nos treadle valves if needed ) I would try to work the valve a bit first. If your truck has been sitting, it may just be stuck. I've never done this, but some guys run a few drops of air tool oil in the air system. This may free up what is sticking. Look at some past threads on here. This has come up a time or two. It's usually an easy fix. (I.e. no parts)

 

[tbar123]

Thanks Planefast, But it's not the treadle valve. I went out and disconnected the smaller air line from the intake. I then plugged it with my thumb and a few seconds later trans fluid started leaking from the pan. Sounds worse too me!

 

[Plane Fast]

I'll have to think about that one. Although the air loss you are having is not from the trans

 

[Plane Fast]

You may just have held enough with you thumb to fine the weak seal in the pan. The trans does move some air.

 

[Plane Fast]

Does the trans vent go to a junction or straight to the intake?
If it's a junction, you are just pressurizing the trans with the leaking air.


I have to be off the computer for a bit. I will say if the truck was running fine before it was parked. It is just a stuck valve. The trucks don't usually just have big failures sitting. Just start with the simple stuff first.

 

[tbar123]

The junction is pictured. 1 line goes to the intake 2 line goes to top of transfer case 3 line appears to be supply.

 

[dilligaf13]

First step: See if the outlet line from the bottom rear of the air compressor is hot to the touch, while the truck is running (200 degrees plus hot). If not the air compressor is not producing enough air. The usual culprit is the unloader head on top of the air compressor. It's a relatively inexpensive and easy fix. If the whole compressor needs replacing they run $300-$800 new and $100-$250 used. Start there. If it's not the compressor is could be the governor or a stuck valve causing air to leak. Let me know the results

 

[Plane Fast]

It sounds like the leak he has is in the intake. The unloader would not cause that type of leak as described. If the unloader is faulty or stuck. It will cause the air dryer to over pressurize and/or have longer then normal blow off.
The other would be slow or no air pressure building. If the ppv was faulty it would cause him to have no wipers and horn. The governor could cause early air cut off. Although he is having a continuous air leak into the air intake. So.... It's down steam of the air dryer and is some where in the brake or transfer case system.

 

[Plane Fast]

This is too funny. It looks like we have been here before!

 

[dilligaf13]

Yup. Funny thing, the solution was in the TMs..... Air leaking out of the stack could be caused by a couple different valves sticking. It depends on if it's leaking when the parking brake is applied or not. One of the issues is the valve on the parking brake. Mine was a valve, I don't remeber which one, under the driver's side step was sticking. Pretty easy fix but a PITA to track down. That's why I usually jump on the brake threads because I know how frustrating they are.

 

[Plane Fast]

I hear ya! Ask me how I know about the unloader..

 

[Coffey1]

I know the q2 valve will dump all air immediately when you step on brake when it's bad. Could be acting different if you're not hitting pedal.
Or the parking brake valve is stuck in the vent release.