Owner manual

3
FIGURE 3 - TRACTOR SERVICE BRAKES APPLYING - TRACTOR TRAILER COMBINATION
SERVICE BRAKES APPLYING - TRACTOR/TRAILER
COMBINATION
Brake application air enters the Bendix
®
AR‑2
™
valve service
port and is conducted through the service piston 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 of its 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 ow out the AR‑2
™
valve delivery ports to the brake chambers.
SERVICE BRAKES HOLDING - TRACTOR/TRAILER
COMBINATION
The air pressure being delivered to the brake chambers
is also present beneath the relay piston. When the air
pressure above and below the 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 chambers is held stable and neither increases nor
decreases.
SERVICE BRAKES RELEASING - TRACTOR/TRAILER
COMBINATION
When the brake application is released, air from above the
relay piston ows back through the proportioning, blend
back and service pistons, to the foot brake valve and is
exhausted. 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 chambers returns to the AR‑2
™
valve and
ows out the open exhaust.
SERVICE APPLICATION - BOBTAIL TRACTOR
When the trailer supply valve (dash control with red
octagonal button) is activated to disconnect the trailer, air
in the AR‑2
™
valve control port and trailer supply line is
exhausted to atmosphere. During bobtail tractor operation,
service application air enters the AR‑2
™
valve service
port and as it passes through the blend back piston, air
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
owing through the blend back piston also exerts a force
on the small diameter of the proportioning piston while
simultaneously owing through the center of it on its way
to the inlet and exhaust valve.
TRAILER
SUPPLY
VALVE
INLET-OPEN
EXHAUST-CLOSED
REAR AXLE
SERVICE
CHAMBER
RELAY PISTON
SERVICE
BRAKE
VALVE
ANTILOCK
CONTROLLER
INLET-OPEN
EXHAUST-CLOSED
BENDIX
®
M-21
™
MODULATOR