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| Illustration 1 | g00301399 |
(1) The secondary brake control valve is located under the cab. | |
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| Illustration 2 | g00301400 |
Service brake control valve (2) Spring (3) Spring (4) Inlet valve seat (5) Outlet port (6) Piston assembly (7) Exhaust port (8) Exhaust valve seat (9) Valve spring (10) Valve body (11) Valve (12) Exhaust diaphragm (13) Inlet port | |
Depressing the brake pedal exerts a force on spring assembly (2) which moves piston assembly (6) downward. Exhaust valve seat (8) then closes exhaust port (7) in valve (11) . Continued downward movement of the piston assembly pushes valve (11) off inlet valve seat (4) . Pressurized air from inlet port (13) passes around the valve and through outlet port (5) . The air flows to the relay valve and tractor brake actuators.
Piston assembly (6) will rise when the air pressure under piston assembly (6) is greater than the spring force. This will permit valve (11) to move upward. Valve (11) will seat against exhaust valve seat (7) . This blocks further supply of pressurized air. Piston assembly (6) remains seated against valve (11) . This prevents loss of air pressure through exhaust port (6) . Thus, the brake control valve is in a balanced position and air pressure is held in the lines and air chambers.
If the pedal is partially raised, mechanical force above piston assembly (5) is decreased. The greater force below piston assembly (6) lifts piston assembly (6) from valve (10) . This unseats piston assembly (6) from valve (10) . Pressurized air in the lines and air chambers is permitted to escape past exhaust diaphragm (12) until the forces are equal on both sides of piston assembly (6) . When the pedal is completely raised, piston assembly (6) will remain unseated from valve (11) . This will exhaust all pressurized air to the atmosphere which will release the brakes. Valve (11) is held seated by spring (9) .