2
NOTE: The force of the spring below the balance piston is
adjustable by an external screw on the modulating
valve. This adjustment is normally made so that
the inlet valve will close at a slightly greater pressure
than the hold-off pressure of the spring brake
actuator, but below maximum system pressure.
OPERATION - NORMAL SERVICE APPLICATION
(FIGURE 4)
When a service application is made by actuating the dual
brake valve, air from the primary circuit is delivered to the
lower side of the control piston through the balance port,
and air from the secondary circuit is delivered to the top of
the control piston through the control port. Because air
pressure from the primary and secondary circuits are not
equal, there will be a slight movement of the control piston.
OPERATION - SERVICE APPLICATION WITH LOSS OF
AIR IN PRIMARY CIRCUIT (FIGURE 5)
A service application made with a loss of air in the primary
circuit would result in reduced air pressure delivered to the
lower area of the control piston. Air pressure from the
secondary circuit on top of the control piston would force
the piston down, opening the exhaust valve and allowing air
pressure in the spring cavity of the spring brake actuator to
release and the spring brake actuator to apply the brakes.
The pressure differential between the primary and secondary
circuits regulates the amount of air pressure released from
the spring cavity of the spring brake actuator. This results
in a total brake application on the rear axle which is
proportionate to the braking on the other axles.
FIGURE 4 - NORMAL SERVICE APPLICATION
SECONDARY
SERVICE
CIRCUIT
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
SINGLE
CHECK
VALVE
BALANCE PISTON
INLET/EXHAUST
VALVE
SPRING
BRAKE
ACTUATOR
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#2 RES.
#1 RES.
PRIMARY -
SECONDARY -
PARK -
FIGURE 3 - CHARGING
SECONDARY SERVICE CIRCUIT
FIGURE 5 - SERVICE APPLICATION LOSS OF PRIMARY
CIRCUIT
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
DUAL BRAKE
VALVE
PRIMARY
SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
SINGLE
CHECK
VALVE
BALANCE PISTON
INLET/EXHAUST
VALVE
SPRING
BRAKE
ACTUATOR
PRIMARY -
SECONDARY -
PARK -
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#2 RES.
#1 RES.
SECONDARY SERVICE CIRCUIT
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK
VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
SINGLE
CHECK
VALVE
BALANCE PISTON
INLET/EXHAUST
VALVE
SPRING
BRAKE
ACTUATOR
PRIMARY -
SECONDARY -
PARK -
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#2 RES.
#1 RES.
FIGURE 6 - SERVICE APPLICATION LOSS OF SECONDARY
CIRCUIT
SECONDARY
SERVICE
CIRCUIT
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
SINGLE
CHECK
VALVE
BALANCE PISTON
INLET/EXHAUST
VALVE
SPRING
BRAKE
ACTUATOR
PRIMARY -
SECONDARY -
PARK -
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#2 RES.
#1 RES.
