BC Transit – Air Brake Course

Secondary Air Loss and Park Brake Function
When a secondary air loss occurs, the two-way check valve will move towards the low pressure side
(secondary reservoir.) This prevents air loss from the primary reservoir into the secondary reservoir.
Because the primary reservoir still contains air pressure, and the two-way check valve has covered up
the secondary reservoir, primary air only will continue to hold the spring brakes in the released
position. This delays the automatic application of the park brake, allowing the operator more time to
choose a safe place to pull over. Also, there is no rear wheel lock-up as occurs in a single circuit system
when air loss occurs.
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