VACUUM CHECK VALVE
ENGINE INTAKE MANIFOLD
ATMOSPHERIC CONTROL LINE
VACUUM VALVE (CLOSED)
RELAY VALVE HYDRAULIC PISTON
RE AY V
PISTON RETURN SPRING
PISTON ROD DRILLED HOLES
(ARROWS INDICATE DIRECTION OF MOTION OF
THE VARIOUS UNITS AND DIRECTION OF
THE FORCES ACTING WITHIN THE HYDROVAC)
MASTER CYLINDER HYDRAULIC PRESSURE
SLAVE CYLINDER HYDRAULIC PRESSURE
Figure 2-44.--Hydrovac operation--applied position.
the atmospheric and vacuum valves in the control
vacuum still present on the front side of both pistons
and atmospheric pressure on the rear sides of both
valve. Thereafter, the slightest foot pedal movement,
pistons, the pistons are forced toward the slave
either toward the released or applied position, will
result in opening either the vacuum or atmospheric
valve, and will partially release or further apply the
Movement of the pistons and push rod toward the
slave cylinder closes the bypass (check) valve, and
then causes the slave-cylinder piston to move outward.
With the exception of the vacuum system and
This forces fluid under pressure into the wheel
particularly the power cylinder, the inspection and
cylinders to apply the brakes. The foot pedal pressure,
maintenance procedures for vacuum brake systems are
acting through the master cylinder, also acts against the
similar to those required for hydraulic brake systems.
slave cylinder piston, assisting the vacuum pistons and
To check the vacuum brake system, first shut off
push rods. The pressure at the wheel cylinders (that is,
the engine and apply the brakes several times to bleed
the total braking effort) is the sum of the output of the
all vacuum from the system. This may require as many
vacuum pistons in the Hydrovac and the foot pedal
as 20 or 30 applications on systems equipped with
pressure at the master cylinder.
vacuum reservoirs. Spongy or soft action of the brake
Release of foot pedal pressure removes the fluid
pedal indicates air in the hydraulic system. If this
pressure from below the relay valve hydraulic piston.
occurs, the system must be bled.
This allows the valve spring in the relay or control
With the pedal held at the normal braking pressure,
valve to return the atmospheric and vacuum valves to
start the engine. If the pedal lowers toward the
the released position. The atmosphere is exhausted
floorboard when the engine starts, the vacuum system
from the rear sides of both pistons, making them
is operating properly. If the pedal fails to move, the
inoperative and allowing the piston return spring to
vacuum system is at fault.
move the pistons to the released position.
Figure 2-45 shows a type of air-over-hydraulic
When the foot pedal movement stops at some
brake system that is used on some of the newer tow
intermediate point between the released and fully
tractors. The air control valve controls the flow of
applied position, the pistons will move slightly toward
compressed air to an air chamber in the master
the applied position. This reduces the fluid pressure
cylinder. The air control valve is mechanically linked
under the relay valve hydraulic piston the necessary
to the brake pedal.
amount to allow the diaphragm to drop and close both