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TM 9-8000
driver for hard braking, the hydraulic pressure in he
causes a stronger braking action. With only a light brake
master cylinder (which operates the valves) causes
pedal pressure, the valves admit less air pressure into
greater valve movement, and therefore, he valves admit
the air-hydraulic cylinder and the braking action is lighter
more air pressure into the air- hydraulic cylinder. This
higher air pressure
source vacuum, the poppet valve in the vacuum check
34-42. Construction. Hydrovac is the trade name for a
valves rests on its seat and, in the event of engine failure
one-unit, vacuum-power braking system. It combines a
or rapid acceleration, traps the vacuum in the Hydrovac
hydraulically actuated control valve, a tandem piston
system in readiness for brake application. As the foot
vacuum- power cylinder, and a hydraulic slave cylinder
pedal is de- pressed, fluid is forced from the master
into one assembly. It is connected hydraulically to both
cylinder through the open bypass (check) valve to the
the master cylinder and the wheel brakes, eliminating the
slave cylinder and on to the wheel cylinders. The fluid
need for mechanical connections with the brake pedal
also is forced through the drilled bypass passage to the
and linkage.
relay valve hydraulic piston, which is forced outward
against the pressure of the valve spring, gradually forcing
The  vacuum-power  cylinder  Is  divided  into  four
the diaphragm plate and vacuum valve seat toward the
compartments by the front and rear pistons and the
applied position. The movement of the diaphragm first
center plate. The vacuum source Is connected directly to
closes the vacuum valve against Its seat, sealing off the
the compartment between the center plate and rear
vacuum from the atmospheric control line. After the
piston. The vacuum Is connected from this compartment,
vacuum valve Is seated, further motion of the diaphragm
by means of the vacuum line, to the relay or control
causes the atmospheric valve to leave Its seat,
valve. From the control valve, the vacuum Is connected
permitting air from the air cleaner to enter the
to the front compartment by a passage In the valve body.
atmospheric control line, then to the compartment
between the center plate and front piston and, through
34-43. Operation. In the released position (fig. 34-41),
the hollow piston rod, to the rear compartment (fig. 34-
with the control valve diaphragm plate and the vacuum
42). With the vacuum still present on the front sides of
valve seat held down by the valve spring to keep the
both pistons, and atmospheric pressure on the rear sides
vacuum valve open and the atmospheric valve closed,
of both pistons, the pistons are caused to move toward
the vacuum is connected through the vacuum valve and
the slave cylinder by the difference in pressure,
atmospheric control line to the compartment between the
Movement of the pistons and push rod toward the slave
center plate and front piston and, through the ports in the
cylinder first closes the bypass (check) valve, and
hollow piston rod, to the rear compartment. Vacuum ,
causes the slave-cylinder piston to move outward,
therefore, is present In all four compartments in the
forcing fluid under high pressure into the wheel cylinders
released position and both pistons remain inoperative.
to apply the brakes. The foot pedal pressure, acting
The piston return spring holds the pistons In the OFF
through the mastercylinder, also acts against the slave-
position. The push rod, in the released position,
cylinder piston, assisting the vacuum pistons and push
maintains the bypass (check) valve off its seat,
rod. The pressure at the wheel cylinders (that is, the total
permitting a direct hydraulic connection from the master
braking effort) is the sum of the output of the vacuum
cylinder, through the hydraulic slave cylinder, to the
pistons In the Hydrovac and of the foot pedal pressure at
wheel cylinders. With this construction, foot pedal
the master cylinder. Release of foot pedal pressure
pressure can be applied to the wheel cylinders for
allows the valve spring In the relay or control valve to
braking action, should vacuum or Hydrovac failure make
return the atmospheric and vacuum valves to the
the  power  cylinder  Inoperative.  The  relay  valve
released position by removing the fluid pressure from
diaphragm has vacuum on both sides and Is held in the
below the relay valve hydraulic piston. The atmosphere
OFF position by the valve spring. When the vacuum In
Is exhausted from the rear sides of both pistons,
the Hydrovac is the same as, or greater than, the


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