Bendix R-7 MODULATING VALVE User Manual
®
Bendix® R-7™ Modulating Valve
SD-03-4504
MOUNTING
HOLE (2)
DELIVERY (2)
MOUNTING
HOLE (2)
BALANCE
BALANCE
PORT
EXHAUST
EXHAUST
CONTROL
PORT
SUPPLY
PORT
SUPPLY
CONTROL
INLET
EXHAUST
VALVE
MOUNTING
SURFACE
INLET
VALVE
SEAT
INNER SPRING
INLET
VALVE
SPRING
BALANCE
PISTON
OUTER SPRING
EXHAUST
CONTROL
PORT
CONTROL
PISTON
DOUBLE CHECK
VALVE (SUPPLY
PORT & BALANCE
PORT)
SINGLE
CHECK
VALVE
DELIVERY
PORT (2)
ADJUSTING SCREW
FIGURE 1 - EXTERIOR & INVERTED VIEW
DESCRIPTION
The R-7™ modulating valve is used in conjunction with a
dual air brake system and spring brake actuators, and
performs four functions:
1. Limits hold-off pressure to the spring brake actuators
(adjustable).
2. Provides for quick release of air pressure from the spring
cavity of the spring brake actuator allowing a fast
application of the spring brake actuators.
3. Modulates spring brake actuator application should a
failure occur in the service brake system.
4. Prevents compounding of service and spring forces.
The valve has one 1/4" control, one 3/8" supply, one 1/4"
balance, two 3/8" delivery NPTF ports, and an exhaust port
protected by an exhaust diaphragm. The valve incorporates
two 13/32" holes for mounting.
FIGURE 2 - SECTIONAL VIEW
Note for model year 2001 and later International Trucks only:
There is an inverted R-7™ valve, which appears inverted from
the other pictures in this document. A standard R-7™ valve
and an inverted R-7™ valve are not interchangeable. See
Figure 1 for a picture of an inverted R-7™ valve.
OPERATION - CHARGING SPRING BRAKE
ACTUATORS (FIGURE 3)
Air pressure used to control the spring brake actuators enters
the valve through the supply port, passing through one side
of the double check valve, through the open inlet valve, over
the balance piston, and out the delivery ports to the spring
brake actuators. When air pressure in the spring brake
actuator cavity has released the spring brake actuators and
when air pressure on top of the balance piston is sufficient
to overcome the force of the balance piston spring, the
balance piston moves allowing the inlet valve spring to close
the inlet valve, shutting off further air pressure from the
reservoir supplying the modulating valve.
1
SPRING
BRAKE
ACTUATOR
PRIMARY SECONDARY PARK -
SECONDARY SERVICE CIRCUIT
CONTROL
PORT
INLET/EXHAUST
VALVE
EXHAUST
PORT
CONTROL
PISTON
BALANCE PISTON
DUAL BRAKE
VALVE
PRIMARY
SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
SINGLE
CHECK
VALVE
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#1 RES.
#2 RES.
PARK
CONTROL
VALVE
SPRING
BRAKE
ACTUATOR
PRIMARY SECONDARY PARK -
SECONDARY SERVICE CIRCUIT
CONTROL
PORT
INLET/EXHAUST
VALVE
EXHAUST
PORT
#1 RES.
CONTROL
PISTON
SINGLE
CHECK
VALVE
BALANCE PISTON
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
#2 RES.
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK
VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
FIGURE 3 - CHARGING
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE
PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
#1 RES.
#2 RES.
CONTROL
PARK
VALVE
SPRING
BRAKE
ACTUATOR
PRIMARY SECONDARY PARK -
SECONDARY
SERVICE
CIRCUIT
INLET/EXHAUST
VALVE
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
SINGLE
CHECK
VALVE
BALANCE PISTON
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
FIGURE 4 - NORMAL SERVICE APPLICA TION
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.
FIGURE 5 - SERVICE APPLICA TION LOSS OF PRIMARY
CIRCUIT
OPERA TION - SERVICE APPLICA TION 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 result s
in a total brake application on the rear axle which is
proportionate to the braking on the other axles.
SECONDARY
SPRING
BRAKE
ACTUATOR
PRIMARY SECONDARY PARK -
SERVICE
CIRCUIT
INLET/EXHAUST
VALVE
CONTROL
PORT
EXHAUST
PORT
CONTROL
PISTON
SINGLE
CHECK
VALVE
BALANCE PISTON
+ SPRING BRAKE HOLD-OFF PRESSURE
- SERVICE AIR PRESSURE
o ATMOSPHERE PRESSURE
FIGURE 6 - SERVICE APPLICA TION LOSS OF SECONDARY
CIRCUIT
#1 RES.
#2 RES.
DUAL BRAKE
VALVE
PRIMARY SERVICE
CIRCUIT
BALANCE PORT
DOUBLE
CHECK VALVE
SUPPLY
PORT
PARK
CONTROL
VALVE
2
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