Line Pressure Relief Valve: Operation

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The line pressure relief valve consists of a steel ball and a spring. The steel ball is pushed against the top end of the valve by spring force.

Fig 1: Identifying Line Pressure Relief Valve
Courtesy of MAZDA MOTORS CORP.
When oil pump discharge pressure (5) (applied to the top end of the valve) becomes greater than spring force, the steel ball is pushed downward, drain port (X) opens and oil pump pressure (5) depressurizes.
If oil pump pressure (5) is depressurized, the steel ball is pushed upward by spring force and drain port (X) closes. This causes oil pump pressure (5) to not depressurize. Thus, the oil pump pressure is adjusted to the fixed pressure.