Operation

Applied position (Fig. 12.20(a)) When the brakes are applied, the air pressure from the foot or hand control valve enters the upper diaphragm chamber, forcing the diaphragm and its central stem down onto the exhaust port seat. The air pressure buildup then deflects downwards the circumferential diaphragm rim, thereby admitting air to the brake actuators via the pipe lines.

Hold position (Fig. 12.20(b)) Movement of air from the inlet port to the outlet ports permits air to occupy the underside of the diaphragm. Once the air pressure above and below the diaphragm has equalized, the diaphragm return spring upthrust pushes the outer diaphragm rim up onto its seat whilst the centre of the diaphragm and stem still seal off the exhaust port. Under these conditions, both inlet and exhaust passages are closed, preventing any additional air flow to occur to or from the brake actuators. The diaphragm is therefore in a state of 'hold'.

Released position (Fig. 12.20(c)) Releasing the air pressure above the diaphragm allows the trapped and pressurized air below the diaphragm to raise the central region of the diaphragm and stem. The trapped air in the brake lines and actuator chambers escape into this atmosphere.

Reducing the brake load slightly decreases the air pressure above the diaphragm, so that some of the air in the brake lines is allowed to escape before the pressure on both sides of the diaphragm balances again. The central region of the diaphragm moves down to close the exhaust port which moves the diaphragm into its 'hold' condition again.

The quick release valve therefore transfers any increased foot or hand valve control pressure through it to the brake actuators and quickly releases the air pressure from the brake actuators when the brake control valve pressure is reduced.

{a> Applied position

Fig. 12.20 (a-c) Quick release valve

{a> Applied position

Fig. 12.20 (a-c) Quick release valve

By these means the air pressure in the brake actuators will always be similar to the delivery air pressure from the brake control valve.

12.3.16 Relay emergency valve (Fig. 12.21(a-d))

Charging (Fig. 12.21(a)) Air delivery from the emergency line (red) enters the inlet port and strainer. The compressed air then opens the check valve, permitting air to flow across to and around the emergency piston, whence it passes to the outlet port leading to the trailer reservoir, enabling it to become charged.

If the reservoir is completely empty, both the relay piston and the emergency piston will be in their uppermost position. Under these conditions, the exhaust valve will be closed and the inlet valve open. Therefore some of the air flowing to the trailer reservoir will be diverted through the inlet valve to the brake actuator chambers, thereby operating the brakes. When the trailer reservoir charge pressure reaches 3.5 bar, air fed through a hole from the strainer pushes down on the annular area of the emergency piston causing the inlet to close. As the reservoir stored pressure rises to 4.2 bar, the downward air pressure force on the emergency piston moves the inlet/exhaust valve stem away from its exhaust seat, enabling the trapped air in the brake actuator chambers to escape to the atmosphere. The brakes will then be released.

Applying brakes (Fig. 12.21(b)) When the brakes are applied, a signal pressure is passed through the service line (yellow) to the upper relay piston chamber, forcing the piston downwards. The lowering of the relay piston and its central exhaust seat stem first closes the exhaust valve. It then opens the inlet valve which immediately admits compressed air from both the emergency line via the check valve (non-return valve) and the trailer reservoir through the central inlet valve, underneath the relay piston and out to the brake actuator chambers. The expanding brake actuator chambers subsequently press the brake shoes into contact with the drums.

Balancing brakes (Fig. 12.21(b and c)) As the air pressure in the actuator chambers builds up, the pressure underneath the relay piston increases its upthrust on the piston until it eventually equals the downward relay piston force created by the service line pressure. At this point the inlet valve also closes, so that both valves are now in a balanced state. Until a larger service line pressure is applied to the relay piston, the central stem will not move further down to open the inlet valve again and permit more air to pass to the brake actuator chambers. Conversely, if the foot brake is slightly released, initially the relay piston is permitted to rise, closing the inlet valve, followed by opening of the exhaust valve to release some of the air pressure acting on the brake actuator chambers.

Releasing brakes (Fig. 12.21(c)) Removing the load on the foot control valve first closes off the air supply to the service line and then releases the remaining air in the service line to the atmosphere. The collapse of service line pressure allows the relay piston to rise due to the existing brake actuator pressure acting upwards against the relay piston. The hollow valve stem immediately closes the inlet valve passage, followed by the relay piston centre stem exhaust seat lifting clear of the exhaust valve. Air is now free to escape underneath the relay piston through the central hollow inlet/exhaust valve inlet stem and out to the exhaust vent flap to the atmosphere. The brake actuators now move to the 'off position, permitting the 'S' cam expanders to release the brake shoes from their drums.

Emergency position (Fig. 12.21(d)) If the air pressure in the emergency line (red) should drop below a predetermined minimum (normally 2 bar), due to air leakage or trailer breakaway, then the air pressure around the upper shoulder of the emergency piston will collapse, causing the emergency piston return spring to rapidly raise the piston. As the emergency piston rises, the hollow inlet/exhaust valve stem contacts and closes the relay piston exhaust stem seat. Further piston lift then opens the inlet valve. Air from the trailer reservoir is now admitted through the control inlet valve to the underside of the relay piston where it then passes out to the trailer brake actuator chambers. The trailer brakes are then applied automatically and independently to the demands of the driver.

A trailer which has been braked to a standstill, caused by a failure in the emergency line pressure, can be temporarily moved by opening the trailer's reservoir drain cock to exhaust the trailer brake actuators of pressurized air.

Do It Yourself Car Diagnosis

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