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TM 9-8000
and holds the pressure constant regardless of Increases
In supply pressure to the valve. In- creases In brake fluid
34-30. Proportioning Valve. Due to the lack of self-
requirement in the closed- off brake line due to heat
energizing and servo effect, disk brakes operate at
expansion or contraction are adjusted automatically by
higher line pressures than drum brakes. For this reason,
the valve through a brief reopening of the valve to supply
a proportioning valve Is used. The proportioning valve Is
pressure. Once the pressure adjustment Is completed,
designed to limit the amount of pressure routed to the
the valve automatically closes off the brake line with the
rear brakes when a combination of disk brakes and drum
limited pressure.
brakes are used on a vehicle. By limiting pressure to the
rear wheels, the chances of rear wheel
lockup are reduced during a hard stop.
34-31. Limiting Valve. The limiting valve closes
off one brake line at a certain brake line pressure
a. Released Position (C, Fig. 34-24). With the brakes
34-32. Construction. Power braking systems are
fully released, and engine operating, the rod and plunger
designed to reduce the effort required to depress the
return spring moves the valve operating rod and valve
brake pedal when stopping or holding a vehicle
plunger to the right. As this happens, the right end of the
stationary. Most power brake systems use the difference
valve plunger Is pressed against the face of the poppet
in pressure between intake manifold vacuum and
valve, which in turn, closes off the atmospheric port. With
atmospheric pressure to develop the additional force
the vacuum port opened, vacuum Is directed to both
required to decrease brake pedal pressure. When a
sides of the diaphragm and the return spring holds the
vehicle Is powered by a diesel engine, the absence of
diaphragm away from the master cylinder In the released
intake manifold vacuum requires the use of an auxiliary
vacuum pump. This type of pump usually Is driven by the
engine or by an electric motor. The vacuum power
b. Applied Position (D, Fig. 34-24). As the brake pedal
booster may be classified into two basic categories:
is depressed, the valve operating rod moves to the left,
vacuum suspended and atmospheric suspended
which causes the valve plunger to move left also. The
fig. 34-24). The  vacuum-suspended power booster
valve return spring then Is compressed as the plunger
utilizes Intake manifold vacuum that acts on both sides of
moves and the poppet valve then comes in contact with
the diaphragm in the released position. When the brakes
the vacuum port seat. As this happens, the vacuum port
are applied, one side of the diaphragm is vented to
to the right side of the diaphragm closes. Continued
atmosphere. This causes the diaphragm to move in the
application of the brake pedal causes the atmospheric
direction of the lowest pressure. This movement
port to open by the valve rod forcing the valve plunger
develops a force that is directed on the push rod of the
away from the poppet. As this happens, atmospheric air
master cylinder to aid in reducing brake
pressure rushes into the control vacuum chamber and
pressure. The atmospheric-suspended booster allows
applies pressure to the hydraulic push rod.
atmospheric pressure to act on the diaphragm in the
released position. When the brakes are applied, the side
c. Holding Position (E, Fig. 34-24). As the driver stops
of the diaphragm  toward  the  master  cylinder  is
depressing the brake pedal, the plunger also will stop
subjected to controlled vacuum, therefore moving the
moving. The reaction of the brake fluid transmitted
diaphragm in that direction, which assists in applying the
through the reaction disk now will shift the valve plunger
brakes. The power brake unit functions during  three
slightly to the right, which shuts off the atmospheric port.
phases  of braking application: brakes
As this position is held, both sides of the diaphragm
brakes being applied, and brakes holding. A typical
contain unchanging amounts of pressure, which
vacuum-suspended power brake unit (B, fig. 34-24) will
be used to describe these operations.


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