Brakes not releasing

I think it is about a 2012 579 but don't hold me to that. Disc brakes on truck. Drum on trailer. Customer was complaining that the brakes were locking up on the trailer. He came in and I realized that with valves pushed in that the air would go to the chambers and brakes apply but they would not release when he took foot off the pedal (truck or trailer). The brake lights on truck and trailer stayed on as well. So I figured it was the little piston froze up in the foot valve. I removed it and cleaned it and lubed and reinstalled but still the same issue. I did push the activator on the foot valve while I had the piston out and it felt like it moved freely. I am thinking maybe the foot valve is bad. I can't think of anything else that would cause both truck and trailer brakes to not exhaust off after taking foot off the brake.

 

I did unhook the blue line and the air released but I did not feel to see which way it came from. Thanks. I will look at in the morning. I think I have one of those valves on the shelf if I need it. Just trying to get some ideas so I can jump on this early in the morning. It might just be the trailer brakes not releasing and not the truck. I did crack the line loose on the steer chamber and I don't think I got air out of it but I am not sure if it released. I didn't feel like rolling around in the dirt and I will not have a stall open until after I finish an A/C job.

 

Well you can unhook the blue line from trailer after pressing the foot pedal and air releases from the blue line but as soon as that air releases no new air comes out. I replaced the foot valve and the inline quick release valve at the blue line connection on rear of the truck and no change. I mashed the brakes and unhooked the air blue line before the quick release and there was air in that line.

 

After testing further only the trailer was messing up. I hooked a pressure gauge into the brake chamber and it released air after letting off the foot valve. Turns out the hand valve was bleeding through the tractor protection valve or not exhausting. It was off but when you mash the foot brake it exhausts some air from it. All I did to solve it was unhooked the exhaust line off of the hand valve. Bendix help line helped me. They said to do this or to replace the tractor protection valve. Since the valve is on nationwide backorder I went this route.