| 摘要 |
A vehicle air brake system includes spring set air released parking brakes to which air is supplied through a pressure holding valve which maintains the air chambers of the parking brakes at a predetermined pressure even though the air brake system may be at operating pressures substantially lower than the predetermined pressure. When the air pressure in the air brake system falls below operational levels, the pressure holding valve responds to release the air from the parking brake chambers. The pressure holding valve is operative to exhaust the parking brake chambers to substantially zero pressure, and is also operative to charge the parking brake chambers any time the input pressure is above zero and exceeds the pressure in the parking brake chambers.
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