Figure 7-23.Tandem-type booster.
In addition to hydraulic system problems, the
brakes may fail to release as a result of a blocked
passage in the power piston, a sticking air valve, or a
broken air valve spring.
Any malfunction occurring in the power booster
will require removing the booster from the vehicle for
repair or replacement. Some power boosters may be
rebuilt or repaired; others are sealed and cannot be
disassembled. Should you have any questions
concerning repairs on the power brake system you are
working on, consult the manufacturers service manual
for proper procedures to follow when testing or
repairing a unit.
The hydraulic-power booster, also called a
hydroboost (fig. 7-24), is attached directly to the
master cylinder and uses power steering pump
pressure to assist the operator in applying the brake
pedal. The hydraulic booster contains a spool valve
that has an open center that controls the pump pressure
as braking occurs. A lever assembly has control over
the valve position and the boost piston provides the
necessary force that operates the master cylinder. See
figure 7-25 for a parts breakdown of a booster
The hydroboost system has an accumulator built
into the system. The accumulator, which is either
spring-loaded or pressurized gas, is filled with fluid
and pressurized whenever the brakes are applied.
Should the power steering system fail because of lack
of fluid or a broken belt, the accumulator will retain
enough fluid and pressure for at least two brake
Parking/emergency brakes are essential to the safe
operation of any piece of automotive or construction
equipment. Parking brakes interconnected with
service brakes are usually found on automotive
vehicles (fig. 7-26). A foot pedal actuates this type of
parking/emergency brake or a dash-mounted handle.