4
FIGURE 4 - TRACTOR SERVICE BRAKES HOLDING - TRACTOR TRAILER COMBINATION
flowing 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 have moved
sufficiently to close its inlet valve without opening the
exhaust.
The inlet remains closed, preventing additional air delivery
to the relay piston and a subsequent increase in brake
chamber pressure, until service application pressure
exceeds approximately 30 psi. Service applications above
this pressure will result in a proportioned increase of the
preset initial brake application to the tractor’s rear brakes.
Proportioning occurs due to the difference in effective area
on each end of the proportioning piston. Tractor rear axle
brake proportioning will occur for all service applications
between approximately 30 and 80 psi.
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 over comes
the resistance of reservoir pressure acting on the large
diameter of the other side. Above 80 psi the ratio between
control and delivered air pressure is reduced and “blends
back” from a proportioned delivery to a full 1 to 1 delivery.
Complete “blend back” to a 1 for 1 delivery is achieved
when a full brake application is made.
SERVICE BRAKES RELEASING - BOBTAIL TRACTOR
When the brake application is released, all air pressure from
between the closed proportioning piston inlet valve and the
AR-2
™
valve service port returns to the brake valve and is
exhausted. Air from above the relay piston, flows back to
the proportioning 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 chambers returns to the
AR-2
™
valve and flows out the open exhaust.
TRAILER
SUPPLY
VALVE
INLET-CLOSED
EXHAUST-CLOSED
REAR AXLE
SERVICE
CHAMBER
RELAY PISTON
SERVICE
BRAKE
VALVE
ANTILOCK
CONTROLLER
INLET-OPEN
EXHAUST-CLOSED
M-21
™
MODULATOR
