Bendix Commercial Vehicle Systems BP-R1 BOBTAIL PROP RELAY VALVE 6/09 - Manual (Page 3)

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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.)
RELAY
PISTON
INLET-
EXHAUST
VALVE
INLET-EXHAUST
PISTON
PROP
PISTON
RELAY
PISTON
INLET-
EXHAUST
VALVE
INLET-EXHAUST
PISTON
PROP
PISTON
SERVICE APPLICATION BOBTAIL TRACTOR
When the trailer supply valve (dash control with red octago-
nal button) is activated to disconnect the trailer, air in the
BP-R1
™ valve control port and trailer supply line is exhaust-
ed 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 founda-
tion brake shoes are brought into contact with the drum.
As service pressure begins to exceed the preset initial
application, the proportioning piston will move suf
fi ciently
to close its inlet valve without opening the exhaust.
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
™ valve
and
fl ows out the open exhaust. (See Figure 6.)
FIGURE 6 - BP-R1 ™ RELEASING - TRACTOR/TRAILER
COMBINATION
FIGURE 5 - BP-R1 ™ HOLDING - TRACTOR/TRAILER
COMBINATION