Air brake help....

[Zachary Cullen]

Morning guys. I'm having a strange issue with my c 7500 Chevy single axle dump. When I set the brakes, everything works as it should, I can release the brakes and it works fine, the next time I set them, not enough air comes off and it takes the spring brakes about 30 seconds to come on. While this is happening, air is slowly leaking from the parking brake quick release valve, I pinch off the line to the maxis and air stops.

If I try to release the spring brakes from the cab, they release but the knob will not stay in but the brakes are release and I can roll the truck. If I step on the brake I am getting air out the service side glad hand.

What I have done-
1- changed the in dash valve, it was leaking a little anyways so no big deal and it helps eliminate one part.
2- I took apart the parking brake quick release valve and found no obvious issue, inspected it and put it back together and reinstalled.

The same issue persists. Where do I look next? Tractor protection valve? I'm kind of at a loss, had a pretty good mechanic look at it the other night and he was stumped as well. I was going to try to bypass the tractor protection valve tonight and see if that would fix the issue, then I can replace it....just hate to throw parts at it and spend a ton of money on what I don't need.

 

[Coaldust]

I would look close at the SR-7, considering the good background description and symptoms you provided.

TopKicks and Kodiaks can be a real PITA to get the correct air brake parts for. No way, GM couldn’t just use off the shelf Bendix stuff. They had to change the standard stuff just enough to make it proprietary. And now it’s not available, except from some shady surplus seller on EBay. So, it creates a temptation to adapt to standard Bendix configuration. But, now you are re-designing a brake system and the liability that accompanies that process. Shame on the mess.

Good luck

 

[Truck Shop]

check your park brake air chambers for internal leakage/feed back.

 

[Zachary Cullen]

Thanks guys I'll check both of these. We checked the chambers but it's worth another look. Either way it's a head scratcher. Hopefully tonight we can dial it in. Any other ideas are appreciated. I will definitely post the solution when we find it.

 

[Truck Shop]

Remove the service line from air chambers with wheels blocked and brakes released. No air should come through that port with emergency line
attached and brakes released.

 

[Zachary Cullen]

Remove the service line from air chambers with wheels blocked and brakes released. No air should come through that port with emergency line
attached and brakes released.

I did this, no air comes out the service side.....issue persists. I can get one cycle to release the brakes then when I set them again they set slowly. Even after they are set, I get air out the glad hands when I step on the service brake

 

[Truck Shop]

If your truck is equipped with a SR7 there is a test procedure in this PFD.
https://n0c357rmy1njbuit2friqwu.blob.core.windows.net/documents/jPcJunINI0EBhB_SD-03-9043_US_000.pdf

 

[Zachary Cullen]

Ok. So I think it's fixed, not sure if there is another underlying issue or not but from the tractor protection valve back, there's one line for each glad hand. The emergency side glad hand had mud daubers in it. I couldn't get air to pass through that. Cleaned it out and now all works as it should.

BUT..... When I charge the tractor side I get a small burst out the emergency glad hand then all air holds. When I release I get a little more air out but all releases normally as it should. I'm thinking the tractor protection valve was still seeing that air pressure and acted as if a trailer was hooked up. Is this normal operation? Also when the spring brakes are released in getting a very very small amount of air out the parking brake quick release valve, does this point at that valve being bad or a different one? It's serviceable now thankfully. Thanks for everyone's help!!!!! Been many hours of head scratching.