M1078 starts losing air when I shift into reverse

Truck airs up fine , shift into drive , everything is good , shift into reverse ..starts losing air fast. Where can I isolate this problem ?

Transmission is not in any way connected to air system. If you are applying brake to shift to reverse, maybe this is what is making you believe the issue is transmission related?

What NDT said, Transmission doesn't interface with the air system. Reverse is a significantly lower ratio than the 2nd gear it shifts into when you go into drive. I am guessing you need to hold more brake pedal in reverse and this is forcing more air out a leak/making a leak worse...

Since this is way easier to search for without the engine running, chock the wheels, apply 120 PSI air to the emergency glad-hand at the front(this releases the park brake) and have someone step on the brakes hard. Use your ears and bubble mix on all the fittings looking for leaks...

thanks , should find it . yeah, I didn't think the transmission had an air connect , but this is what is happening. I'm getting some time to solve a few issues with the trucks, so I'm on it !

I find the tractor valve on the back frame rail is a common problem. You get a crack in one of the diaphragms and air escapes from the valves vent when braking. And the 3 valves under the drivers foot well area can cause air leak when braking. Same issue they get cracked diaphragms.