M 1084A1 Air venting when park brake valve disengaged

[Mad Deranger]

My park brake valve is venting when air is supplied to release the park brakes. Parking brake works when applied. Air continues to vent with the engine off and the Park Brk. off. I rummaged through the postings here and got a student's grasp of the air system. Thanks to Ronmar for posting the schematic at https://www.steelsoldiers.com/threads/m1078-drives-through-brakes.202833/

My thinking is that I have a back feed from the down side of the valve, if not the valve is bad. Would I be correct in thinking that I have a bad brake diaphragm or anti-compound valve problem? Would service line air pressure work against park air if there is a leak?

 

[Suprman]

Where is the air venting from? You could have a bad brake chamber. Hole in the spring brake diagram.

 

[Ronmar]

Exactly, where is the air venting from?

If up behind the grill, that vent is also shared by the treadle valve vent, so you need to determine which device is venting. The treadle valve vents from a port on the rear facing side of the valve body above the pedal. It has a line you can easilly disconnect that runs around thru the dash to the port in front of the radiator To decide if it or the park control valve is the source of the venting. there may be an even easier way to confirm this…

If it is the treadle venting, and it only does it with park air applied, you may have a bad anti-compounding valve. The anti compounding valve basically has a 2way check valve built into it’s input. It accepts either park air or secondary air from the treadle valve to release the park springs, so you don’t get both park spring and service brakes applied at the same time. That 2way may be leaking park air back to the treadle which with no pedal applied is venting that air. Stepping on the pedal and applying air pressure = to park air should stop the venting if this is the case…

 

[Mad Deranger]

Taking the panel off today revealed that the trailer supply valve is leaking at a fitting. It turned out to be a minor issue (famous last words). Still, thanks to you guys, I learned something more about my truck. Now, how do I get a wrench in there???

 

[Ronmar]

Taking the panel off today revealed that the trailer supply valve is leaking at a fitting. It turned out to be a minor issue (famous last words). Still, thanks to you guys, I learned something more about my truck. Now, how do I get a wrench in there???

Wrench? Oh thats funny… pick the shortest line you can reach with a wrench and remove it. That will allow you to pull it out a little farther to get to the next shortest, and so on. Keep doing this till you either reach the one that is leaking or have it de-installed for replacement oh, and don’t forget which is which, dots of colored paint or sharpee marks on bodt and hoses can help here…

 

[Mad Deranger]

I have a set of crows feet for 25 years that I have never used.... till today. And yes. The paint pen is a marvelous thing. Thanks again for the advice.

 

[Mullaney]

I have a set of crows feet for 25 years that I have never used.... till today. And yes. The paint pen is a marvelous thing. Thanks again for the advice.

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Crows Feet are magic. Used to work on a lot of corner mounted derricks and the only way to get to the hydraulic hoses was with "an old crow". They do help you understand how tight is tight enough when you use them. Good and snug is more than enough when addressing JIC fittings...