Question:
All trucks, truck tractors, and buses must be
equipped with emergency brakes and parking
brakes. They must be held on by mechanical force
(because air pressure can eventually leak away).
Spring brakes are usually used to meet these
needs. When driving, powerful springs are held
back by air pressure. If the air pressure is removed,
the springs put on the brakes. A parking brake
control in the cab allows the driver to let the air out
of the spring brakes. This lets the springs put the
brakes on. A leak in the air brake system, which
causes all the air to be lost, will also cause the
springs to put on the brakes.
(According to CDL Handbook)
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