Air leaking into air intake stack (parking brake NOT applied)

So I'm getting air leaking into the air intake stack but it's when the parking brake is released, not when the parking brake is applied (as indicated in the TM). This is causing the primary and secondary air systems to bleed down. When I activate the parking brake the leaking stops and air pressure in both systems builds right up to normal. I've replaced the treadle valve, with another unit from a working truck. Reading the TM the only other thing that it lists is the one-way check valve. I'm going to try and pull it off and take a look at it tomorrow. It's obviously hard to tell, when the truck is running, but it almost sounds like air is escaping somewhere near the parking brake. I've hooked shop air up but it wasn't much help tracking the issue down. Anyone else have a similar issue? Thanks.

The only thing I can suggest is to let the air pressure build completely, then then shut the truck down, then use a piece of long flexible air hose( I use 1/4") stick one end in your ear and use the other end to search for the leak. When you come across the leak you will definitely hear it.I hope this helps!

Scroll down and read some of te similar threads.

ONE of your SPRING BRAKE rubbers is leaking, what you here is air leaking into the vent system, the easiest way to find which pot is leaking is to remove the vent line from the pots, then see which POT is leaking air, DO NOT check the line because the939s use a common vent system.

I checked all the Service Brake Chamber's vent lines and there was no air leaking at any of them so their out as a cause.

dilligaf13 said:

I checked all the Service Brake Chamber's vent lines and there was no air leaking at any of them so their out as a cause.

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I did NOT say service brakes, I SAID " spring brake RUBBER", air holds the spring brake off, no air the spring brake sets. You HAVE a spring brake rubber leaking.

Gotcha. Nothing in the -23 TM about that, guess I'll have to do a little more research.

So I unhooked all the vent lines, from the four rear spring brake chambers and no air was leaking from any of the fittings. In the attached picture the bottom line is the supply and the others are vent lines, best I can tell from the TM pictures. I think that removes them as the causes of the air leaking into the intake stack.

Did you remove the vent lines with the spring brake set or released, In post number 1, you say that you lose air when the spring brakes are released, If you are SURE none of the pots are leaking, look at the quick release valve that is under the left cab step.
When the spring brakes are set, do you still lose air if you apply the service brakes ??

73m819 said:

Did you remove the vent lines with the spring brake set or released,

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73m819 said:

I removed the vent lines with the spring brakes set, spring brake release control valve out for normal operation, and there were no leaks. I don't remember if I checked for leaks, from the vent lines, with the spring brake release control valve in.

In post number 1, you say that you lose air when the spring brakes are released,
I lose air, through the stack, when the parking brake is released.

If you are SURE none of the pots are leaking, look at the quick release valve that is under the left cab step.
Will do. Can the parking brake valve malfunctioning cause this issue? Nothing in the TM about it but.....

When the spring brakes are set, do you still lose air if you apply the service brakes ??
I'll have to triple check tonight, when I get home, but I'm 99% sure I remember that I don't lose air when I apply the service brakes. Thanks.

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Try pushing on your brake pedal slightly and see if it quits. Mine was the treadle valve assembly.

Ddmk18 said:

Try pushing on your brake pedal slightly and see if it quits. Mine was the treadle valve assembly.

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Second that mine had a slight air leak in the stack and i pushed slightly on the pedal and it stops

No luck with the brake pedal trick plus I replaced the treadle valve and pedal with a unit from a working truck.

I checked for air coming from the brake chamber vent lines, with the spring brake release control valve in, and the was none. The air was still leaking into the stack though.

My truck leaks down though intake also.
I also have checked brakes can not find no problems.
So another member and I have been talking about this issue.
We at this time think the air is leaking into transfer case and pushing gear oil up into the vent line and into filter box
How quick does your truck bleed down.

It'll leak down in a few minutes, if I leave the parking brake down. With the parking brake engaged it'll hold air overnight or longer.

Then I would think that valve on the hand brake needs lubed or rebuilt.
The last couple times this has come up that valve would not release the spring brake because it would not go to the vent portion of the valve
Once it was oiled with air gun or WD 40 it worked fine

Check the quick release air brake valve in the driver side step. It was no good and was bleeding into the breather.

So I removed the vent line on the control directional linear valve, on top of the parking brake lever. According to the TM once the parking brake lever is released air should come out of this vent line. As no air is coming out of the vent line the TM says to replace the quick release valve. I pulled mine apart a couple days ago, when 73m819 suggested it was the problem, and put some air tool into it but it appears as though it was and is still in-op. I'll try to pick one up tomorrow and put an end to this saga....

It was in fact the quick release valve causing all the problems. In retrospect the first clue should have been the lack of air escaping through the air stack (farting) when I engaged the parking brake. The more I work on my M932 the more I like my M813.....


Air Brake QR-1 Quick Release Valve For M939 Series, 288251

Product Description

QR-1 air brake quick release valve for M939 series 5 ton trucks. P/N 288251 (11602155). NSN 4820008572737.

The function of the quick release valve is to speed up the exhaust of air from the air chambers. It is usually mounted close to the chambers it serves. Crack pressure: 1 psi. All ports are 3/8 inch N.P.T.

dilligaf13 said:

It was in fact the quick release valve causing all the problems. In retrospect the first clue should have been the lack of air escaping through the air stack (farting) when I engaged the parking brake. The more I work on my M932 the more I like my M813.....


Air Brake QR-1 Quick Release Valve For M939 Series, 288251

Product Description

QR-1 air brake quick release valve for M939 series 5 ton trucks. P/N 288251 (11602155). NSN 4820008572737.

The function of the quick release valve is to speed up the exhaust of air from the air chambers. It is usually mounted close to the chambers it serves. Crack pressure: 1 psi. All ports are 3/8 inch N.P.T.

Click to expand...

Is that the QR valve under the driver steep?