overnight air loss

[davidbrady]

Al,

Here's what a pressure protection valve looks like: .

Another common place for leaks are the drain valves for the air tanks (wet, primary, secondary, and accessory). Drain valves are located here:

Also, have you performed a DOT brake test? Start the engine, build to full pressure, turn off engine, apply the brakes, let pressure stabilize for one minute, hold for an additional two minutes, pressure should not drop more the 4 psi per minute.