All vehicles are equipped with a pressure gauge (10) to indicate the amount of air pressure in the main reservoir system. This gauge is mounted in the cab, usually on the dashboard. Common operating pressures are 85 P.S.I. to 105 P.S.I. and 105 P.S.I. to 125 P.S.I., depending on the system. Monitoring the gauge will alert the driver to any unusual changes in pressure.