M934 Air Leak

[captcarc]

Hi Heavy Iron Friends.

I haven't had time to really dig into this air leak problem, but wanted to write where I am with it and seek advise and direction. My M934 with ABS takes longer all the time to build-up air pressure but can be coaxed with revs to reach 120# on primary and secondary. When engine is shut down pressure will return to 0# within 5 minutes.

With the engine shut down, I hear a pulsing sound seemingly coming from the front of the primary tank and feel vibration there. If I close the valve at the wet tank, all noise stops. With that valve closed, I can quickly build and retain primary pressure while secondary remains at 0#. I have sprayed soapy water on many areas/connections and can't locate what must be a large leak. In the meantime, is it safe to drive the truck with the wet tank valve closed, primary pressure fine and secondary at 0#?

Thank you in advance for your awesome help as always.

 

[simp5782]

Download the p2p program. It will break it all down for you. Which valve did you close on the wet tank? Front at the pressure protection valve? Or on the side/top? The pressure protection valve opens up all secondary items such as wipers. Horn. Etc thru the air gov. You may also have a leaking diaphragm on your brake chambers. . They get aired up thru the primary tank.

It is all Depending on which valve you closed

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[captcarc]

Thank you simp5782! I'm fairly sure that I closed the valve on the top/center of the wet tank. Where do I find the p2p program?

 

[simp5782]

That is secondary then. Charge your air system and chock your wheels. Release your emergency brake and see if it drops with the tank open.

If so apply brakes and hold and count how many seconds it takes to drain the secondary.

Your main suspect is the Q2 valve up in front of the front axle. Especially a blend off that quick. It is dumping it into the intake thru that valve. Especially if it dumps the secondary air in under 20 seconds.

Its in the TMs

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[captcarc]

Thanks! I seem to lose pressure faster with the emergency brake on, then off, and definitely lose air pressure fast with foot brake depressed. I get air "honking" in the intake whenever I depress the foot brake and can hear the air escape into the intake when I hold it down. However, I already replaced the Q2 valve with a suspect unit off epay on which I had to tap a drilled-only hole on the top of it. Perhaps I should replace it again to be sure if you know a good source. Also, could a bad Q2 cause dramatic pressure loss with truck just sitting there, not running, emerg brake off? And can I drive the truck with the wet tank valve closed?

 

[porkysplace]

[h=1]TM's for the 939 series trucks[/h]

M939_P2P.zip

 

[M35A2-AZ]

Air compressor to charge the truck and spray bottle with soapy water will find lots of air leaks. I have to get my wife out to listen for air leaks, I can not hear them.

 

[simp5782]

Did you put the lines back on right? The two drivers middle and lower one. Switch them. And see what happens.

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[simp5782]

Also check replacing your two air brake diaphragms.

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