The air going back to the towed vehicle is NOT the air from the Prevost brakes. The air from the Prevost brakes controls how much air is past from the "New Air Reserve Tank" to the "Towed Vehicle" in the above diagram. If the area between the "New Air Reserve Tank" and the "New Relay Valve" is compromised, air will be able to escape from the coach's reserve tank. That is where the "New Pressure Protection Valve" comes in. When the pressure drops in the "Existing Primary Air Reserve" to ~80-90 psi., the "New Pressure Protection Valve" closes, allowing the breach to be contained, and the coach's brakes to operate safely. Since the air pressure in the "Existing Primary Air Reserve" is lower than where the coach wants it, the compressor builds more air. Any air above the pressure protection valve's minimum limit is allowed through (to atmosphere in this case). This is where the sound of "air" would be coming from, and why the coach was still able to operate safely.
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