| 摘要 |
A parking brake control valve for a vehicle fluid pressure braking system is actuable by the vehicle operator to control communication from a fluid pressure source to supply hold-off pressure to the vehicle parking brakes. The parking brakes are of the spring applied, fluid pressure released type, in which the spring actuator automatically effects a mechanical brake operation when hold-off pressure drops below a predetermined level. The parking brake control valve is responsive to a supply pressure below a predetermined level to automatically vent the hold-off pressure from the parking brakes, but this predetermined low pressure is less than the value of the spring brake hold-off pressure below which the brakes are gradually applied or "drag". The valve disclosed in the present application includes a check valve which isolates the hold-off pressure from supply pressure so that the hold-off pressure cannot vent in response to a decrease in the supply level to a sufficiently low value where the brakes may "drag". However, when the supply pressure drops below the predetermined low pressure at which the spring brakes are to be applied, the parking brake control valve automatically vents the hold-off pressure to effect a spring brake application. Consequently, the system disclosed in the present application prevents the spring brakes from "dragging", while retaining the safety feature of automatically applying the spring brakes when the supply pressure drops below the predetermined level.
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