The air brake safety valve blows off at 150 psi (1034 kPa) because the pressure in the system is too high.
Most air brake safety valves are set to release excess pressure from the system at 150 psi (1035 kPa).
This increase poses a danger to the component parts of the system.
As well, the compressor in the system is capable of pumping air pressure to 500 psi (3450kPa), which poses danger to both the driver and others around the vehicle.
It is the governor that controls the cut-in (load phase) and cut-out (unload phase) pressures of the compressor.
If the pressure in the system is too high, it is most likely the governor that is faulty, but it could also be the compressor.
The driver of a commercial vehicle must know that the safety valve makes the sound of a machine gun when it releases excess air pressure from the system.
It makes a distinct sound so the driver knows that a problem exists.