Figure 3-49.-Typical vacuum brake cylinder.
results in much greater hydraulic pressure than air
pressure admitted to the air cylinder. Valve action varies
with the amount of pressure applied to the brake pedal.
When heavy brake pedal pressure is applied by the
operator for hard braking, the hydraulic pressure in the
master cylinder (which operates the valves) causes
greater valve movement. As a result, the valve admits
more air pressure into the air-over-hydraulic power
cylinder and this higher air pressure causes a stronger
In a vacuum brake system, depressing the brake
pedal opens a valve between the power cylinder, which
contains a piston, and the intake manifold to which the
power cylinder is connected (fig. 3-49). When you apply
the brakes, air is exhausted from the cylinder head of the
piston. At the same time, atmospheric pressure acts on
the rear side of the piston to exert a powerful pull on the
rod attached to the piston.
When the brake valve is closed, the chamber ahead
of the piston is shut off from the intake manifold and is
opened to the atmosphere. The pressure is then the same
on both sides of the piston; therefore, no pull is exerted
upon the pull rod. The brakes are released and the piston
returned to its original position in the power cylinder by
the brake shoe return springs.
HydrovacTM is a trade name for a one-unit vacuum
power-braking system. It combines a hydraulic control
valve, a vacuum power cylinder, and a hydraulic slave
cylinder into one assembly. This assembly (fig. 3-50) is
connected to both the master cylinder and the wheel
brakes and eliminates the need for mechanical
connections with the brake pedal.
Pressure on the brake pedal forces fluid from the
master cylinder through the check valve to the slave
cylinder and to the wheel cylinders. Also, the foot pedal
pressure, acting through the master cylinder, acts also
against the slave cylinder piston to help the vacuum
pistons and pushrods to press against the brake shoes.
Figure 3-50.HydrovacTM power brake cylinder.