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| Illustration 1 | g00526684 |
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Brake air relay valves (1) Pressure protection valve for the brake air system (2) Brake air relay valve that is for the Automatic Retarder Control (ARC) (3) Brake air relay valve for service brakes and for manual retarder | |
The brake air relay valves are located on the top of the frame behind the fuel tank. Brake air relay valve (3) activates all four of the brake master cylinders when either the service brake is applied or the retarder is applied. Brake air relay valve (2) activates all four of the brake master cylinders when the brakes are applied by the Automatic Retarder Control (ARC). The pressure protection valve for the brake air system (1) prevents a complete loss of air pressure in the event of a failure of the relay valve for the ARC.
Brake air relay valve (3) receives signal pressure when the service brake pedal is pushed or when the manual retarder is used. Brake air relay valve (2) receives signal pressure when the Automatic Retarder Control (ARC) is used.
When the brake pedal is pushed or when the manual retarder is used, signal pressure is sent to the control passage of the brake air relay valve (3). This causes an internal valve in brake air relay valve (3) to open. This allows air from the primary air tank to flow through the brake air relay valve (3). This air flows to the air chambers of the brake master cylinders. The brake master cylinders then engage the brakes.
When the ARC is used, signal pressure is sent to the control passage of the brake air relay valve (2). This causes an internal valve in brake air relay valve (2) to open. Air flow from the primary air tank flows through the pressure protection valve for the brake air system (1) until the air pressure reaches the cut-in pressure. When the air pressure reaches the cut-in pressure, the pressure protection valve for the brake air system (1) opens. This allows air to flow through the brake air relay valve. This air flows to the air chambers of the brake master cylinders. The brake master cylinders then engage the brakes.
Brake air relay valves (2) and (3) both release in the same way. When the brake pedal is released or when retarding is not needed, the signal pressure to the control passage of the brake air relay valve will stop. The internal valve in the brake air relay valve will shift in order to block the flow of air from the primary air tank. When the brake master cylinders do not receive air flow to the air chambers, the brakes are released.
There is a double check valve at the front master cylinders and there is a double check valve at the rear master cylinders. The double check valves allow brake air relay valve (2) and brake air relay valve (3) to share the same air lines to the brake master cylinders.