Air wont build up bleeds off thru intake

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tobyS

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With the sound of air coming out the intake, it sounds like a stuck (open) pressure relief on the compressor head.
 

simp5782

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With the sound of air coming out the intake, it sounds like a stuck (open) pressure relief on the compressor head.
Not possible. the compressor gets its air from the aftercooler side of the engine. Plus it wouldnt be able to vent to the stack when the truck is running.

It's a bad linear valve. More than likely if it goes away with the handle off. I had it happen on a 931 awhile back and I just swapped to the push pull type. Just needed to run thru diagnostics with the most likely to the most non likely to eliminate what it wasn't. Most people almost assume it's a q2 valve when it vents to the intake but it can be alot of other stuff. Seen them pressurize the transfer case and make them puke oil
 
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aztec

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air leak to intake with em brake handle off when I engage pull up air leak stops removes lineal valve and seemed to work fine pushed spring break over ride button in on dash to override spring breaks and air leaked into intake has air in tanks
 

simp5782

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The linear valve is venting when it is off. The spring brake override does the same thing as taking the emergency brake off so should yield the same result. You can crimp the little line off that is the vent for the linear to see if it stops.
 

simp5782

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So you found the issue with the linear valve or do we need to proceed further?

If you need to proceed further. I am going to suspect a bad relay valve and/or a bad service diaphragm. on the service brakes on the inside between the axles remove the line on the cap end and see if you have air pressure with the parking brake released. Start with the drivers front tandem. The both drivers side service brakes are applied thru the front relay valve. this is under the truck between the axles. A bad relay valve can keep constant air and blow a diaphragm which returns to the intake as vented air from the system even though its pressure. If nothing on the drivers side then try the passengers. I had a stuck R8 valve the other day. It can happen from a hard stomp on the pedal.

If you have not done any PM brake stuff then I suggest replacing all your spring and service brake diaphragms
 

silverstate55

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When that happened to my M931A2 right after I bought it from GL it turned out to be the interlock cylinder on the transfer case...the large air piston was rusted in place, and the air dryer was badly in need of a rebuild (letting moisture pass through).

Simp5782 is the one to closely follow his instructions, especially his videos...if he can't walk you through a repair, nobody can....
 
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