
AIR BRAKES AND AUXILIARY RETARDERS 451
pressurized air via a hand control valve, the latter being
described later. Since air pressure is reduced to apply a
spring brake actuator and increased to apply an air
brake actuator, the air brake engineer distinguishes
between the two forms of air supply by referring to
them as inverse air and upright air respectively.
Although air pressure is released from a spring brake
for brake application, there still remains sufficient
compression in the expanded spring to exert the re-
quired force for actuation of the brake. A spring brake
may be either applied gradually by the hand control
valve for the purpos ...