COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
AIR DRYER - Installed in supply line to wet tank and removes moisture from inlet air to wet tank (M939/A1 air
dryer kit installed only).
AIR COMPRESSOR - Draws in air from the intake manifold and forces it into the brake system and wet tank
SAFETY VALVE - Located at the inlet side of the wet reservoir, it prevents pressure build-up by releasing air
pressure exceeding 150 psi (1034 kPa) when the governor fails to regulate air supplied by the compressor.
WET TANK RESERVOIR - Performs two functions:
Traps water in air reservoir to protect other air systems from freezing or corroding.
Stores reserve air supply enabling operator to make normal stops when engine stalls or compressor fails.
PRESSURE PROTECTION VALVE - Performs two functions:
Allows air pressure to build to 60-65 psi (414-448 kPa) before supplying air to auxiliary air-powered
Closes off auxiliary air system from other systems if an accessory fails and prevents loss of air from
ALCOHOL EVAPORATOR (M939/A1 series) - Helps protect air lines from freezing.
WET TANK RESERVOIR DRAINVALVE - Provides a drain for moisture and air from reservoir.
AIR DRYER - Removes moisture from inlet air to wet tank (M939A2 only).
EXPELLO VALVE - Augments air dryer condensation blowdown by venting moisture when compressor cycles.
GOVERNOR - Trips valve inside compressor to regulate flow of air to the system. When pressure builds to 120-
127 psi (827-876 kPa), the governor will close valve.