Page 172 - Driving Commercial Vehicles Manual+

chapter 8 — air brakes

The most common type of parking brake in an air brake system is the spring
parking brake. The second type is known as a safety actuator and is usually
found only on some highway coaches and intercity buses.

Spring parking brakes

Most spring parking brakes consist of an additional chamber attached to the
rear of a service brake chamber. The added chamber contains a powerful coil
spring arranged so that the spring force can be applied to the brakes through
the normal service chamber pushrod.

A spring parking brake
chamber attached to a
service brake chamber.
The brakes are off.

fast fact This diagram shows the main components of a typical combination spring and
service brake chamber.

The spring parking brakes Spring parking brakes are mounted on the rear axles only — not on steering
will apply if air pressure falls axles. The service brake chamber contains the normal pushrod, diaphragm
beneath a certain level, and return spring. The spring parking brake section is mounted behind the
but their effectiveness in service brake chamber.
stopping the vehicle or
holding a parked vehicle
depends on how well the
brakes have been kept in
adjustment.

This concrete mixer has
spring brakes on its tandem
rear axles only — not on the
twin steering axles, nor on
the booster axle at the rear.

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