
AIR BRAKES AND AUXILIARY RETARDERS 445
loading and allows excess air to be vented to the
atmosphere through the exhaust port. The safety valve
remains closed during all normal operation of the air
brake system.
26.3 SYSTEM CONTROL
Foot-operated brake valves
These are treadle operated. Their function is to provide
control of system air pressure, during application and
release of the brakes which is precise and proportional
to driver effort. Consider first a single foot brake valve.
This essentially comprises a treadle-operated telescopic
plunger and piston assembly, which acts upon a combi-
nation inlet and exhaust valve at its foot. The plunge ...