Bendix Commercial Vehicle Systems R-8P User Manual
®
Bendix® R-8P™ Pressure Equalizing Pilot Relay Valve
SD-03-1061A
SPRING
VENT
INLET/EXHAUST
VALVE ASSEMBLY
SUPPLY PORT
FIGURE 1 - CUTAWAY AND EXTERNAL VIEW
EXHAUST
DESCRIPTION
PISTON
PISTON
ORIFICE
SERVICE
(CONTROL)
PORT
DELIVERY
PORTS
OPERATION
COVER
BODY
SUPPLY PORT
DELIVERY PORT
The R-8P™ pressure equalizing pilot relay valve is a device
designed to reinforce the control signal on long wheel base
towing trailers and converter dollies.
Unlike a standard relay valve which typically has a 4 p.s.i.
crack pressure and similar differential between control and
delivery, the R-8P™ valve has a 0 p.s.i. differential.
On the converter dolly and long wheel base towing trailers,
the R-8P™ valve reinforces the service brake application
signal without the differential found in standard relay valves.
The 0 p.s.i. differential of the R-8P™ valve assures that all
trailer and dolly brakes will receive the same strength control
signal even during light brake applications.
By providing a balanced pressure signal to all trailer and
dolly brakes, the R-8P™ valve helps achieve overall vehicle
brake balance and stability. The R-8P™ valve also helps
improve brake lining wear balance on combination vehicles
by causing even the most distant brake to contribute its
share of braking effort.
APPLICATIONS (FIG. 2)
When a service brake application is made, air delivered from
the brake valve enters the service port of the R-8P™ valve.
As air pressure fills the space above the piston, it begins to
force the piston down. Because of the spring force above
the piston, the piston exhaust seat is already in contact
with the inlet/exhaust valve, sealing off the exhaust. As the
piston begins to move downward it forces the inlet/exhaust
valve off the inlet seat, allowing air from the reservoir to be
delivered out the delivery ports of the R-8P™ valve in direct
proportion to the pressure received at the service port.
Any differential between service and delivery pressure caused
by friction or resistance to the movement of the piston
(commonly called crack pressure and normally in the range
of 2 to 4 p.s.i.) is eliminated due to passage of air through
the orifice in the piston.
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PISTON
R-8P
VALVE
™
SPRING
PISTON
ORIFICE
BRAKE
VALVE
PISTON
R-8P
VALVE
™
SPRING
PISTON
ORIFICE
BRAKE
VALVE
SERVICE
SUPPLY
SERVICE
INLET/EXHAUST
VALVE ASSEMBLY
RESERVOIR
VENT
DELIVERY
FIGURE 2 - OPERATIONAL VIEW OF APPLICATION
RELAY
VALVE
BRAKES
BALANCE (FIG. 3)
The air pressure being delivered by the open inlet valve also
is effective on the bottom area of the R-8P™ valve relay piston.
When air pressure beneath the piston equals the service air
pressure on top of the piston, the piston lifts slightly and the
inlet/exhaust valve is returned to the inlet seat. The exhaust
remains closed as the delivery pressure balances with the
service line pressure. Balancing of the pressure above and
below the piston through the orifice of the R-8P™ valve piston
assures that the service and delivery pressures are equal
and 0 p.s.i. differential exists. As delivered air pressure to
the R-8P™ valve service port is changed, the valve reacts,
holding the desired application pressure.
EXHAUST OR RELEASE (FIG. 4)
When air pressure is released from the service port and air
pressure in the cavity above the piston of the R-8P™ valve is
exhausted, air pressure beneath the piston lifts it upward,
further seating the inlet/exhaust valve against the inlet seat.
Raising the exhaust seat of the piston off the inlet/exhaust
valve, opens the exhaust. This allows the delivery air pressure
to rapidly exhaust to atmosphere. As air pressure under the
piston drops, the spring force above the piston will push the
piston downward, once again placing the exhaust seat of
the piston in contact with the inlet/exhaust valve. Any pressure
remaining under the piston will pass through the piston orifice
and be exhausted at the brake valve.
PREVENTIVE MAINTENANCE
Important: Review the Bendix Warranty Policy before
performing any intrusive maintenance procedures. A warranty
may be voided if intrusive maintenance is performed during
the warranty period.
SERVICE
SUPPLY
EXHAUST
RELAY
VALVE
RESERVOIR
INLET/EXHAUST
VALVE ASSEMBLY
FIGURE 3 - OPERATIONAL VIEW OF BALANCED POSITION
™
R-8P
VALVE
PISTON
SUPPLY
RESERVOIR
INLET/EXHAUST
VALVE ASSEMBLY
FIGURE 4 - OPERATIONAL VIEW OF RELEASE
DELIVERY
SPRING
PISTON
ORIFICE
EXHAUST
RELAY
VALVE
DELIVERY
VENT
SERVICE
BRAKES
BRAKE
VALVE
SERVICE
SERVICE
BRAKES
No two vehicles operate under identical conditions, as a
result, maintenance intervals may vary. Experience is a
valuable guide in determining the best maintenance interval
for air brake system components. At a minimum, the valve
should be inspected every 6 months or 1500 operating hours,
whichever comes first, for proper operation. Should the valve
not meet the elements of the operational tests noted in this
document, further investigation and service of the valve may
be required.
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