BC Transit – Air Brake Course

In this diagram, the dark green line is a LARGER diameter than the bright
green line. The dark green line delivers reservoir air to the foot valve AND to
the relay valve at the rear axle. When a brake application is made, the foot
valve delivers the requested amount of air to the relay valve, through the
bright green line, known as a “control line.”
The air delivered by the foot valve “dead-ends” at the relay valve. This causes
the relay valve to “open” and it delivers the requested amount of application
air from its own supply of reservoir air.
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Control line
Reservoir air
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