BENDIX SD-13-4796 Fiche De Données

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 flow out the AR‑2

™

 

valve delivery ports to the brake chambers.

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.

COMBINATION
When the brake application is released, air from above the 

relay piston flows 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 

flows 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 

flowing through the blend back piston also exerts a force 

on the small diameter of the proportioning piston while 

simultaneously flowing 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