4
RELAY
PISTON
INLET-EXHAUST
VALVE
INLET-EXHAUST
PISTON
PROP
PISTON
FIGURE 8 - BP-R1
™
CONTROL VS. DELIVERY PRESSURE
RELAY
PISTON
INLET-EXHAUST
VALVE
INLET-EXHAUST
PISTON
PROP
PISTON
Service applications of more than 80 psi cause the blend
back piston to begin to move. Service air pressure acting
on the full diameter of the blend back piston overcomes the
resistance of reservoir pressure acting on the large diam-
eter of the other side. Above 80 psi the difference between
control and delivered air pressure is reduced and “blends
back” from a proportioned delivery to a full 1:1 delivery.
Complete “blend back” to a 1:1 delivery is achieved when
a full brake application is made, and it will occur regardless
of reservoir pressure level.
SERVICE BRAKES RELEASING — BOBTAIL
TRACTOR
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 between the closed proportion-
ing piston inlet valve and the service piston returns to the
exhaust diaphragm. If the release is rapid, the diaphragm
fl exes in response to the returning air and opens the
exhaust.
Air from above the relay piston, fl ows back to the pro-
portioning piston causing it to move. As it moves, the
proportioning piston unseats the exhaust valve allowing
air from above the relay piston to escape to atmosphere.
Reducing the air pressure above the relay piston causes
pressure beneath it to lift the piston away from the exhaust
valve. Air from the service brake actuators returns to the
BP-R1
™
valve and fl ows out the open exhaust.
As service air pressure continues to increase, the inlet valve
opens as necessary to deliver additional but reduced air
pressure to the relay piston, thus to the rear axle brakes.
Proportioning occurs due to the difference in effective area
on each end of the proportioning piston.
FIGURE 7 - BP-R1
™
HOLDING - BOBTAIL TRACTOR
0 PSI
0 PSI
TRACTOR/
TRAILER
COMBINATION
BOBTAIL
TRACTOR
BRAKE VALVE DELIVERY PRESSURE
(BP-R1
™
VALVE CONTROL PORT)
SER
VICE BRAKE CHAMBER PRESSURE
(BP-R1
™
V
AL
VE DELIVER
Y POR
T)
FIGURE 9 - BP-R1
™
RELEASING - BOBTAIL TRACTOR
