M923 Air leak problem

[Tackettr]

Gentlemen,
My M923 losses air from the secondary tank when the brake pedal is held, like at a stop light. It will bleed down and activate the low air warning system. There is a hissing sound from the intake stack while this is occurring. I have check all lines and diaphrams/canisters and all are good. Any help is appreciated.

 

[gimpyrobb]

Glad hands out back turned on?

 

[Tackettr]

I wish it was that simple. Somehow air pressure is getting to the vent lines only while the brakes are depressed.

 

[ranchhopper]

You have a diaphram going out the 923 has the exhausting air from the brake chambers plumbed back into the intake the air is passing through the hole in the diaphram and exhausting out into the intake.

 

[Tackettr]

Update: This problem turned out to be a bad diaphram. The problem with locating it for me was not having a helper. I would have to start the truck to build pressure, then place a brace on the brake pedal then jump out and try to back off the vent line on a particular wheel prior to loosing all the air, then repeat. Well somewhere during one these processes I followed the wrong rabbit hole and missed the bad diaphram. That is why I was thinking it was something else, like the brake pedal valve.

After posting here I decided to start over, however this time I would hookup my shop's air compressor to the emergency glad-hand to eliminate the repeated in and out of the truck. This worked great and allowed me to quickly identify the bad diaphram. I recommend this method for everyone trying to locate air leak problems.

Thanks for the help guys.

 

[flibob]

Now that is good thinking! Nice job and thanks for passing it along on SS.

 

[Moosemcnally]

I have the same problem. The primary side leaks down and we can hear it in the stack. The primary means it is in the rear wheels correct?

 

[MtnSnow]

Moose, Most likely the Q2 valve found on the plate that is in the center of the truck just below the front bumper

 

[Moosemcnally]

31 M900 Series Air Check Valve NAPA MBI KN23000
Is this the right item off of the parts list?

 

[MtnSnow]

No that is not the part you need.

The part you need is a Bendix LQ2 from Napa or much much cheaper on the Big E site

 

[Monkeyboyarmy]

Moose, Most likely the Q2 valve found on the plate that is in the center of the truck just below the front bumper

Yep. About $30......LQ-2 quick release valve. You may want to take it apart to see if it can be easily repaired.

 

[Moosemcnally]

Thanks everybody!!