[77newell]

To clarify Richard's description of the PPV at least as it applies to the one on my coach. The PPV allows flow through the valve whenever the inlet pressure is above a certain value, on my coach it is set at about 67psi. On our coaches it allows air to be used for non-airbrake uses only when above its set point. Therefore, as Richard described, helping to keep the brakes operating normally should an air leak larger than your compressor can make up occur in the non-airbrake parts of the coach compressed air system. If the PPV were not in place, a rupture in the supply line to the 6-pack for example would leak air fast enough that the pressure in the air brake system would drop to the point that your emergency/parking brakes would automatically engage. The PPV prevents that from happening.