cat 615c brakes

[precision dirt]

I know most scrapers don't have much for brakes but i road mine alot so I need decent brakes. My cab side tractor brake stays engaged at startup and only way to get it to release is pull plug out of air chamber but then i don't have service brakes on that side. Right now i have to install plug, start machine and air up, release brakes ( at this time cab side brake stays engaged) until I remove plug. I can run this way but would rather not road machine. All over service brakes hold but are limited on pressure because of whats escaping system on this chamber. I have replaced diaphragm and cleaned double check valve on chamber.
Could i have a problem on the relay valve? Possible stuck assembly on one port? I have never been into one of these air relays on this machine and wanted a few opinions for the ones that have some experience.

 

[Mark250]

Hi, there are two sources for air at the double check valve on the service brake booster. one is via the service brake relay and the other is via the secondary brake pilot valve .to determine which circuit is causing the problem you need to isolate one or the other.
follow the air lines from the brake booster one will go to a relay valve the other will go to a tee. build up air pressure and push in your secondary brake valve in the dash remove the air line going into the tee there should be no air at this point.
if there is, the secondary pilot air valve is allowing air though ,either because it is faulty or because the secondary brake control valve is leaking
check this first and get back to me
Mark
serial number please

 

[precision dirt]

Ok thanks. Im just getting to it this morning. I will let you know what I find. Thanks for your help.
s/n 46Z00872

 

[precision dirt]

I finally got it clean enough to start work on it, after some thought if secondary pilot is leaking wouldnt all brakes be held or dragging? Right now it only seems to be cab side front brake affected. What i have found is that once i start up and release brakes (cab side still holding) I have to pull plug from chamber to release. After release i can install plug and use service brakes as normal, if i pull secondary pilot valve out and then try to release brakes will lock until I go through plug procedure again. Kinda sounds like secondary pilot could be culprit?