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Does it leak when you are not stepping on the pedal? Service brake system really only gets air when the pedal is pressed. If its leaking out of the passenger side front gladhand that could be the pictured check valve. Or the tractor valve. This should probably be moved to the lmtv forum you...

Im not feeling spraying oil into the air system. There are several components with plastic diaphragm valves that are prone to cracking. I would take off the check valve behind the bumper that was previously pictured and clean that first.

There is another valve behind the bumper also it's a service brake quick release valve. It can be cleaned sometimes they get stuck.

That part is a check valve and a common culprit. If air bleeds out all over though, the tractor valve may need a rebuild.