M977 Brake issue

[Rockymyers]

Let me start off by saying please forgive my lack of proper terminology. I have a 1983 M977 HEMTT, that I love by the way, that has never giving one bit of trouble. When I hopped into it last time it fired up, but when I released the parking brake air started leaking out from behind the brake peddle. This flow of air will drain the air tanks quite fast making it impossible to move. I have asked a few big rig mechanics about it, and apparently this system is just different enough that all of the trouble shooting techniques they suggested did not work for me. Anyone who can tell me which direction I need to go in would be appreciated!

 

[fuzzytoaster]

Start by downloading all the TM's for your truck under the "TM" tab top right of the website header. They're great guides.

Secondly, when you press the parking brake button in how much air is being released? Does it hold pressure until you touch the brake pedal? It certainly sounds like something is getting stuck and reminds me of the Q2 valve and treadle valves on the M939 series. Builds pressure but one tap to the brakes and it dumps the tanks. I'd start looking at the 24P-1 page 192 & 192-1 which is that valve on your HEMTT.

 

[simp5782]

Post a picture of the place it's leaking

 

[charlesmann]

Sounds like the treadle valve failed.

 

[Rockymyers]

Here is what happens. When the truck is started the air pressure builds up as it should and levels off. Once you release the parking brake switch on the dash air will come out of the back of the valve block that is at the top of the brake peddle. I cannot see exactly where the air is coming out, but it is definitely behind that block. It will drain the air tanks quickly. If you push hard, and I mean hard, on the brake peddle the air will stop leaking allowing the tanks to build up pressure again.

 

[Karl kostman]

I have to go with the treadle valve either failed completely or is stuck. I would start with the TMs to see if there is anything for trouble shooting this valve and if not go to the section that explains the removal of the valve once out see if you can take it apart for cleaning and lubrication and remember our 977s weren't built yesterday stuff like this is going to happen! By the way I love mine also!