BENDIX SD-13-4796 Fiche De Données

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 Bendix

®

 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

BENDIX

®

 M-21

™

 

MODULATOR