Troubleshooting guide

26
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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 benefi ts 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 the 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 relay valve models are available in
multiple “crack pressures”. The crack pressure value for
a particular valve varies 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 Bendix
®
R-14
®
relay valve incorporates both
an integral double check valve, with a balance port
connection which provides both an anti-compounding or
quick-exhaust feature, depending upon vehicle.
The Bendix
®
BP-R1
®
bobtail proportioning relay
valve is a combination of two valves in a single housing.
The lower portion contains a “standard” service brake
relay valve, which functions as a relay station to speed up
brake application and release. The upper portion houses
a brake proportioning valve which reduces normal
service brake application pressure when the tractor is not
towing a trailer (bobtail operation). The control port on
the Bendix BP-R1 valve is connected to the trailer supply
valve delivery and signals bobtail operation application.
The anti-compound feature is used when these valves
are used to control spring-actuated parking features.
The Control System: Relay Valves
Bendix
®
R-12DC
®
Relay Valve
(Straight
trucks)
Bendix
®
R-12P
™
Relay Valve
(0 crack pressure for
long wheelbase trailers
and dollies)
Bendix
®
R-8P
™
Relay Valve
(0 crack pressure for
long wheelbase trailers
and dollies)
Bendix
®
R-12
®
Relay Valve
Bendix
®
R-14
®
Relay Valve
(Anti-compounding)
Bendix
®
R-6
™
Relay Valve
Bendix
®
BP-R1
®
Bobtail
Proportioning Relay Valve