BC Transit – Air Brake Course

Dual Air System Summary
1) The purpose of dual air is to shorten stopping distance, reduce brake failures, and split
the system so that if one side fails, the other works
2) When secondary air is lost, the front brakes are sacrifice. They are less important because
most of the braking is accomplished by the rear brakes.
3) When primary air is lost, the spring brakes are employed as replacements for the nonfunctioning
rear service brakes. The modulator valve “senses” the primary air loss and
begins to “modulate,” or vary, the brake pressure of the spring brakes in accordance with
the front service brake application.
4)Whenever a system failure occurs, the operator must pull over to the side of the road and
stop the unit. Even if the compressor is pumping, it is pumping out to atmosphere, so the
system cannot be replenished.
5) Since the air brake system is split into two parts, each part (primary or secondary) is
capable of keeping the spring brakes released. Remember the two-way check valve? This
delays the automatic application of the park brake allowing the operator more time to select
a place to pull over and stop.
6) Under normal operation, the air from both primary and secondary systems “blend” at the
two-way check valve. One-way check valves are found at the inlet of the primary and
secondary reservoirs. …2
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