Vacuum Systems
When the brake is not pressured, excess hydraulic fluid
is returned to the hydraulic reservoir.
Air has weight (atmospheric pressure), and this
The major difference in this system is that it has an
weight results in a pressure of approximately 14.7 psi
open hydraulic system pressurized to 2200 psi with a
at sea level. It is this air pressure that is used in the
600 pound precharge in the accumulator to provide
operation of vacuum brake systems.
braking power even when the engine is not running.
It is impossible to create a perfect vacuum, but by
(Good for a limited number of uses.) The rest of the
pumping air from a container, it is possible to obtain
brake components are similar in appearance and
several pounds per square inch difference in pressure
operation to those of any disc brake system.
between the outside and the inside of the container. If
the container were suddenly opened, outside air
Power Brake Systems
(atmospheric pressure) would rush into the container
The brake system most commonly used on
to equalize the pressure. It is upon this principle that
powered support equipment is the hydraulically
the power cylinder of a vacuum brake system operates.
operated system. However, the increase in size and
There are many varieties of vacuum-powered
weight of aircraft has required heavier and more
brake systems, and it is impossible to cover them all
p ow e r f u l s u p p o r t e q u i p m e n t , e s p e c i a l l y t h e
within the scope of this course. However, the system
aircraft-towing tractors. With this increase in vehicle
most commonly used on support equipment is called
power and weight, it is necessary to have a brake
Hydrovac.
system that is more effective and less strenuous for the
The Hydrovac combines into one assembly a
operator. It was almost impossible for the operator to
hydraulically actuated control valve, a tandem piston
apply sufficient braking action to control a heavy, yet
vacuum power cylinder, and a hydraulic slave cylinder
comparatively small, vehicle. To compensate for this,
(fig. 2-42). The vacuum power brake cylinder is
some hydraulic brake systems are equipped with a
connected hydraulically to both the master cylinder
form of power system to assist the force of the
and the wheel cylinders.
operator's foot in applying the brakes.
The vacuum source for this system, as for all
Power brake systems use the principle of the
vacuum brake systems, is the intake manifold of the
hydraulic brake to operate the wheel brake cylinders
engine. Reciprocating engines, particularly gasoline
and produce braking action. In addition, these systems
engines, generate a substantial vacuum in this area
use the energy of air pressure, either to apply the
except when running at full power. A check valve
necessary pressure to the hydraulic fluid or to assist in
maintains a vacuum within the system, even after the
this application. Atmospheric pressure provides this
engine is stopped, by closing the intake manifold when
energy in some power systems, while a compressor is
the pressure in the manifold rises above the vacuum
required in others. Many of the Navy's aircraft-towing
pressure within the system. A vacuum reservoir is
tractors are equipped with some type of power brake
usually required so that a substantial source of vacuum
system. Some are equipped with a form of
is available. Once air is pumped out of the vacuum
va c u u m - b o o s t s y s t e m , w h i l e o t h e r s u s e a n
air-over-hydraulic system.
reservoir through the intake manifold, the resulting
HOSE CONNECTIONS
BLEEDER SCREWS
CONTROL
VALVE
END
PLUG
SLAVE CYLINDER
VALVE CYLINDER
VACUUM POWER
FITTING PLUG
CYLINDER
ASf02042
Figure 2-42.--Hydrovac vacuum power brake cylinder.
2-34
