Spring brake release (Fig. 12.18(a)) When the vehicle is in motion with the brakes released, the upright valve assembly inlet valve is closed and the exhaust valve is unseated, permitting all the air in the trailer brake actuators to be expelled. Conversely the inverse valve assembly delivers a signal pressure to the spring brake relay valve. This results in the line from the secondary/park reservoir to the tractor spring brake actuators to be open. Thus a large volume of air will be delivered to the air chambers controlling the compression of the power springs and the releasing of the tractor brakes.

Secondary brake application (Fig. 12.18(b)) As the handle is moved across the gate to make a secondary brake application it rotates the cam, depressing the upright plunger. The exhaust valve closes and the inlet valve is unseated, causing compressed air to pass to the trailer brake actuator chambers. As the pressure in the brake actuators increases, the air pressure acting on top of the upright piston causes it to move down against the upthrust exerted by the graduating spring, closing the inlet valve. This procedure is repeated for further handle movement until the full secondary brake position is reached when the air pressure delivered to the trailer brake chamber is at a maximum.

During this operation the inverse valve assembly, which was delivering maximum pressure when the handle was in the 'off' position, is exhausting

(b) Secondary brake application

Fig. 12.18 (a-c) Spring brake hand control valve until with the secondary brake position the delivered pressure is zero.

In other words, the upright valve delivers a gradually increasing pressure to the trailer brake actuators and, at the same time, the inverse valve assembly allows the air pressure on the tractor spring brake actuators to be gradually released.

Park brake application (Fig. 12.18(c)) When the handle is moved from the secondary brake position to the park position, the cam lifted by the leverage of the handle about its pivot allows the upright plunger and the inverse plunger to be raised. The air pressure in both tractor and trailer brake actuators then exhaust into the atmosphere. The tractor brakes are now applied in the park position by the mechanical force exerted by the spring actuators.

12.3.14 Relay valve (piston type) (Bendix)

Purpose The relay valve is used to rapidly operate a part of a braking system when signalled by either a foot or hand control valve. This is achieved by a small bore signal line feeding into the relay valve which then controls the air delivery to a large bore output service line. As a result, a small variation in signal pressure from the foot or hand valve will produce an instant response by the relay valve to admit air from the service reservoir directly to the service line brake system.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook

Post a comment