Back to diagnosing brake problems on Merric's 76 RD. First thing, Alex, I checked for air leaks from the maxibrake chambers. I've got an electronic stethoscope that can hear a fly fart at a hundred paces, so I blocked the wheels, aired everthing up and released the maxis. Not a peep, and believe me, the tiniest air leak sounds like a hurricane when I crank the gain up on this rig. Then to Keith's suggestion re TP valve. The trailer this truck pulls is a 20 ton tandem equipment trailer, pintle hitch, dates from the early 80s, no spring brakes. It has the old style emergency/parking system - when the supply air drops below 75 psig, it traps the air in the trailer and applies the service brakes. The trailer was completely rebuilt two years ago. We stripped off the deck, cut out the old crossmembers, tipped it on its side and sandblasted every inch, welded in all new steel, painted and new 2 1/2" oak deck. It was completely rewired and got a spindle- out brake job. Everything works. We aired the trailer, ran the guage pressure up to 120 and I popped the supply gladhand. Pressure was still over 90 when the TP valve cycled and shut off the air. So good news is, the TP valve is good. Bad news, I still don't know where the air is coming from at the service gladhand, trailer air shut off and treadle valve applied. It's not much - at moderate foot valve application, I can stop the air flow at the gladhand with my thumb. But it's there, and the truck would fail a roadside inspection. There's a dummy on the gladhand, so it's not losing air, but it ain't quite legit. I think my next step is to check the dash valves for leakes and bypassing. This truck has the old three valve system from the first FMSS 121 days, tractor park, trailer park and system park (part of the system designed by geniuses with too much time on their hands and no grasp at all of the real world of trucking). The 121 air control alves under the truck had been replaced when Merric bought the truck, so I'm dealing with conventional air flow there. Everything at the dash seems to work fine, but ya never know, might find something. I'm eventually (key word there) going to find the problem with this thing, and any advice from anyone out there that speeds up the process will be greatly appreciated. I live in hope. Thanks, Mark