As soon as the vehicle is started (in neutral), main pressure is established by the ______________.

• A. Priority valve
• B. Hold regulator valve
• C. Spring-loaded MPRV
• D. Trimmer regulator valve

Answer: (C)

Main pressure in the air brake system is controlled by the spring-loaded Main Pressure Regulating Valve. This valve regulates how much air from the compressor is allowed into the main reservoir, holding back flow until the system reaches the set pressure. When you start the vehicle, the compressor begins delivering air and the MPRV opens to charge the main line up to the governor’s cut-out point, then maintains that pressure by modulating flow. The priority valve directs air to service lines at higher pressures but doesn’t set the main reservoir pressure itself, and the hold regulator and trimmer regulator valves serve different regulatory functions rather than establishing the initial main pressure.