Troubleshooting guide

23
www.bendix.com 1-800-AIR-BRAKE (1-800-247-2725)
Relay Valves
Relay valves are primarily used on vehicles to apply
and release rear axle(s) service or parking brakes.
When the driver applies the brakes, air travels through
the delivery (in this case signal) line to the relay valve
and moves an internal piston down. This closes the
exhaust and opens the delivery of air to the brakes.
The primary benefits of using a relay valve is that the
high capacity of air needed for braking is delivered
directly and does not have to travel up to the brake
valve and then all the way to the brakes. The brake
force is adjustable and when released the relay valve
exhausts to atmosphere. Relay valves are generally
mounted close to the chambers they serve and are
available in both remote and reservoir mount designs.
The inlet/exhaust valve cartridge can be replaced
without line removal.
In order to design braking systems with good brake
timing, that is, where brake application occurs in the
correct sequence, some models of relay valves are
available in multiple “crack pressures”. The crack
pressure value for a particular valve is varied by the
use of different strength return springs within the valve
which the air pressure signal must overcome before
air begins to be delivered to the brake chambers.
The R-8
™
and R-14
™
relay valves both incorporate
an integral double check valve with a balance port
connection which provides both an anti-compounding
or quick exhaust feature depending upon vehicle
application. The anti-compound feature is used when
these valves are used to control spring actuated parking
features.
R-7
™
Modulating Valve
The R-7
™
Modulating Valve is used in dual circuit
brake systems and performs four functions; during
normal operation, it limits hold-off pressure to the spring
R-7
™
Valve
Spring Brake Actuator
R-7
™
Valve
Park
Control
Valve
brakes, provides quick release of air
pressure from the spring cavity of
the spring brake actuator allowing
a fast application of the spring brake
actuators, modulates spring brake
actuator application should a failure
occur in the service brake system
and prevents compounding of
service and spring forces.
R-12DC
™
Relay Valve
(Straight
trucks)
R-12P
™
Relay Valve
(0 crack pressure for
long wheelbase trailers
and dollies)
R-8P
™
Relay Valve
(0 crack pressure for
long wheelbase trailers
and dollies)
R-14
™
Relay Valve
(Anti-
compounding)
Relay Valves
R-12
™
Relay Valve
R-6
™
Relay Valve
R-8
™
Relay Valve
(Anti-compounding,
frame or reservoir mounted)