BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio descargar pdf (Pagina 16)

SERVICE BRAKES RELEASING

Tractor/Trailer Combination

When the brake application is released, air pressure in the service port of the BP-R1

returns to the foot brake valve and is exhausted. Air from above the relay piston flows

back through the proportioning, blend back and service pistons to the exhaust

diaphragm. If the service brakes are rapidly released, the diaphragm flexes in

response to the returning air and opens the exhaust. If the brakes are slowly released,

service air pressure will flow through the slot behind the diaphragm and back to the

brake valve. 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 actuators returns to the BP-R1 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 BP-R1 control port and trailer supply line is exhausted

to atmosphere. During bobtail tractor operation, service application air enters the

BP-R1 service port, causing the exhaust diaphragm to seal the exhaust port. Service

application air passing through the blend back piston exerts a force on the full effective

diameter of the piston. The blend back piston remains stationary at application pres-

sures 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. Once past the inlet and

exhaust valve, service air pressure exerts a force on the larger diameter end of the pro-

portioning 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 move sufficiently to close its inlet valve

without opening the 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.

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 the reservoir pressure, acting on the large diameter of the

other side. Above 80 psi the difference between control and delivered air pressure is

reduced and “blend 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.

Air Brake System

Revised 11/15/99

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BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio Pagina 16

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