3
FIGURE 4 - CUTAWAY VIEW OF RELEASED POSITION
AIR INLET
NON-APPLIED POSITION
OPERATION
Brakes Applied
During a brake application, modulated air pressure from the
foot operated air brake valve enters the actuator inlet port.
Air pressure acting upon the area of the actuator diaphragm
causes the diaphragm and push rod assembly to move
toward the master cylinder. The actuator push rod forces
the master cylinder piston to move within its bore. Initial
forward travel of the piston moves the actuating lip of the
piston away from the stem of the compensating (tilt) valve
allowing it to close due to spring force. (Refer to Figure 2 &
3.) Closure of the compensating valve shuts off the fluid
passage between the cylinder bore and fluid reservoir which
allows hydraulic pressure to build and the brakes to apply.
The amount of hydraulic pressure generated is dependent
upon the air pressure applied to the air actuator.
Brakes Released
When air pressure is exhausted from the actuator the push
rod returns to the released position due to spring force.
With the push rod load removed, fluid pressure combined
with return spring force causes the piston to return to the
brakes released position. The actuator lip of the piston
contacts the stem of the compensating valve and unseats
the valve as the piston returns. When the compensating
valve opens, the passage between the fluid reservoir and
master cylinder bore is also open. Any excess fluid
remaining at the end of the stroke due to “pumping” and/or
volume change due to temperature fluctuation is transmitted
back to the fluid reservoir as the compensating valve opens.
The push rod is used as the piston stop in the fully released
position. (Refer to Figure 4).
Fluid Level Indicator
The reservoir fluid level is reduced as the brake linings wear
due to an increase of fluid volume required in the hydraulic
brake system. The float contained in the reservoir filter cap
assembly is suspended in the reservoir fluid and responds
to changes in the fluid level. As long as the reservoir fluid
level is adequate, a metal contact plate on top of the float
stem is held away from two electrical contacts. When the
reservoir fluid level drops below the “MINIMUM” fluid level
mark on the reservoir, the contact plate on the float stem
will move and complete the electrical circuit through the
switch.
DIAPHRAGM
NON-APPLIED POSITION
COMPENSATING TILT
VALVE UNSEALED
POSITION
HYDRAULIC
FLUID PORT
RETURN SPRING
NON-APPLIED POSITION
PISTON
NON-APPLIED
POSITION
