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1) The valve inlet of the brake control valve is covered by dirt; The return spring of the valve is too soft, rusted or stuck with oil.
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2) The sliding surface of the valve body and plunger seat is strained or poorly lubricated, resulting in blocked valve closure.
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3) The rubber sealing ring on the plunger seat expands, so that the upward moving force of the plunger seat is greater than the pre-tightening force of the valve return spring, and can not be closed.
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4) The axial vent on the valve is blocked by oil or dirt, so that the upper and lower chambers of the valve body are isolated, and the lower chamber forms a vacuum.
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2. Troubleshooting
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If the symptoms disappear when the brake pedal is pressed, this is the fault. Removable plunger seat assembly, clean surface dirt and oil, or replace bad parts. At the same time, the sliding surface of the valve body and plunger seat should be coated with a small amount of disulfide, and the loading test should be carried out