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.