BENDIX BP-R1 User Manual
Bendix® BP-R1™ Bobtail Proportioning Relay Valve
SD-03-1067
EXHAUST
SUPPLY
CONTROL
SERVICE
DELIVERY
CONTROL
SERVICE
EXHAUST
SUPPLY
DESCRIPTION
The BP-R1™ bobtail proportioning relay valve is a combination of two individual valves in a single housing. The lower
portion, or body , of the BP-R1
service brake relay valve, which functions as a relay station
to speed up brake application and release. The upper portion, or cover, houses a brake proportioning valve, which
reduces normal service brake application pressure when
the tractor is not towing a trailer.
Used in tractor air brake systems only, the BP-R1
is installed in place of a standard relay valve. It functions
as a standard service relay during operation of a tractortrailer combination. During bobtail tractor operation, the
BP-R1™ valve improves controllability and reduces stopping
distances by reducing air pressure to the rear actuators/
chambers, whose axle load has been greatly reduced.
While the BP-R1™ valve can incorporate various crack
pressures to accommodate specific applications, the
standard is 4 psi.
™
valve contains a “standard”
™
valve
FIGURE 1 - BP-R1™ BOBTAIL PROPORTIONING RELAY VALVE
Designed for frame or cross member mounting, the BP-R1™
valve is furnished with a mounting bracket which is attached
to two of the four cover cap screws. Replacement valves
are furnished with a universal mounting bracket, which
allows the BP-R1™ valve to replace most standard
relays.
All air connections on the BP-R1™ valve are identifi ed with
cast, embossed letters for ease of installation. The letter
identifi cation and air line connections are shown below
for reference.
BP-R1™ Valve Air Connection Embossed Identifi cation
Supply (to reservoir) SUP
Delivery (to brake actuator) DEL
Service (brake valve rear axle service delivery) SER
Control (to delivery of trailer supply valve) CON
1
TRAILER
SUPPLY
TRACTOR
PROTECTION
CONTROL
PORT
“BLEND BACK”
PISTON
SERVICE
PORT
SERVICE PORT
BP-R1™
VALVE
CONTROL
PORT
DUAL
BRAKE
VALVE
REAR AXLE
SERVICE RESERVOIR
FIGURE 2 - BP-R1™ TRACTOR AIR SYSTEM
SUPPLY
DELIVERY
PORT(4)
SERVICE
SPRING
BRAKE
OPERATION
INITIAL CHARGING
During the initial build up of tractor system air pressure,
supply air fl ows through internal body and cover passages
in the BP-R1™ valve to the blend back piston.
PROPORTIONING
PISTON
INLET-EXHAUST
VALVE
SPRING
(NOT USED
IN ALL
MODELS)
SUPPLY
PORT
INLET-EXHAUST
VALVE
FIGURE 3 - BP-R1™ BOBTAIL PROPORTIONING RELAY
VALVE
SERVICE
PISTON
EARLIER
MODELS HAVE
A DIAPHRAGM
HERE
SPRING
RELAY
PISTON
DELIVERY
PORT
of the inlet and exhaust valve. With the exhaust passage
sealed, continued movement of the piston unseats the inlet
portion of the inlet and exhaust valve, allowing supply air
from the reservoir to fl ow out the BP-R1™ valve delivery
ports to the brake actuators. (See Figure 4.)
Activating the trailer supply valve (dash control with red
octagonal button), simultaneously charges the trailer and
the BP-R1™ valve control port. Air entering the BP-R1™
valve control port causes the proportioning piston to move
toward its stop in the cover. The integral inlet and exhaust
valve is carried along with the piston until it seats and
seals the exhaust passage. Continued movement of the
proportioning piston opens the inlet valve. With the tractor
and trailer air system fully charged, the vehicle may be
operated.
SERVICE BRAKES APPLYING — TRACTOR/
TRAILER COMBINATION
Brake application air enters the BP-R1™ valve service port
and is conducted through the service piston to the exhaust
diaphragm which fl exes in response to the incoming air and
seals the exhaust passage in the exhaust piston. Air fl ows
around the exhaust diaphragm then through the center of
both the blend back and proportioning pistons to the inlet
and exhaust valve. Flowing around the inlet valve, application air moves through a passage in the cover to the top of
the service relay piston. In response to air pressure, the
relay piston moves into contact with the exhaust portion
INLET-EXHAUST
VALVE
FIGURE 4 - BP-R1
COMBINATION
PROP
PISTON
INLET-EXHAUST
PISTON
™
APPLYING - TRACTOR/TRAILER
RELAY
PISTON
2
SERVICE BRAKES HOLDING — TRACTOR/
TRAILER COMBINATION
The air pressure being delivered to the brake actuators is
also present beneath the relay piston.
When the air pressure above and below relay piston is
equal, the piston moves slightly allowing the inlet valve to
return to its seat. The exhaust valve remains closed. With
both the inlet and exhaust valves closed, air pressure in
the brake actuators is held stable and neither increases
nor decreases. (See Figure 5.)
PROP
PISTON
INLET-EXHAUST
VALVE
PROP
PISTON
INLET-EXHAUST
VALVE
RELAY
PISTON
INLET-EXHAUST
PISTON
RELAY
PISTON
INLET-EXHAUST
PISTON
FIGURE 5 - BP-R1
COMBINATION
™
HOLDING - TRACTOR/TRAILER
SERVICE BRAKES RELEASING — TRACTOR/
TRAILER COMBINATION
When the brake application is released, air pressure in the
service port of the BP-R1™ valve returns to the foot brake
valve and is exhausted. Air from above the relay piston fl ows
back through the proportioning, blend back and service
pistons to the exhaust diaphragm. If the service brakes are
rapidly released, the diaphragm fl exes in response to the
returning air and opens the exhaust.
If the brakes are slowly released, service air pressure will
fl ow through the slot behind the diaphragm and back to
the brake valve. As air pressure is reduced above the relay
piston, pressure beneath it lifts the piston away from the
exhaust valve and opens the exhaust passage. Air from
the service brake actuators returns to the BP-R1
and fl ows out the open exhaust. (See Figure 6.)
™
valve
FIGURE 6 - BP-R1™ RELEASING - TRACTOR/TRAILER
COMBINATION
SERVICE APPLICATION BOBTAIL TRACTOR
When the trailer supply valve (dash control with red octagonal button) is activated to disconnect the trailer, air in the
BP-R1™ valve control port and trailer supply line is exhausted to atmosphere. During bobtail tractor operation, service
application air enters the BP-R1™ valve service port
causing the exhaust diaphragm to seal the exhaust port.
Service application air passing through the blend back
piston exerts a force on the full effective diameter of
the piston. The blend back piston remains stationary at
application pressures below approximately 80 psi because
of opposing reservoir air acting on the large diameter.
Application air fl owing through the blend back piston also
exerts a force on the small diameter of the proportioning
piston while simultaneously fl owing through the center of
it on its way to the inlet and exhaust valve. Once past the
inlet and exhaust valve, service air pressure exerts a force
on the larger diameter end of the proportioning piston,
which opposes the air pressure and spring force exerted
on the other end. The proportioning piston inlet valve
remains open until a preset, initial application pressure
has reached the relay piston assuring that the foundation brake shoes are brought into contact with the drum.
As service pressure begins to exceed the preset initial
application, the proportioning piston will move suffi ciently
to close its inlet valve without opening the exhaust.
3
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