Think First, Then Troubleshoot is a 12-part series of articles by Richard J. Mitchell describing a systematic approach to diagnosing problems with hydraulic systems. Here is Chapter 11: Pressure-reducing Valves
Chapter 11: Pressure-reducing valves
Pressure-reducing valves (PRVs) maintain secondary, lower pressures in branches of hydraulic systems. (Upstream main system pressure is still determined by the system relief valve or other pressure-setting device.) Pressure-reducing valves are normally open, 2-way valves that allow system pressure fluid to flow through them until a set pressure is reached downstream. They then shift to throttle flow into the branch.
Pressure-reducing valves are actuated by forces exerted by pressure downstream. These forces establish the desired working pressure by creating a pressure drop across the valve’s spring-biased main spool. A pressure-reducing valve is not an on-off device: the position of its main spool adjusts continuously to maintain the desired pressure setting. Of all pressure-control valves, pressure-reducing valves are the most sensitive to contamination-related malfunctions.
A pressure-reducing valve can malfunction in a variety of ways. To troubleshoot a pressure-reducing valve, refer to Figure 14 and install pressure gauges to read inlet pressure at test port TP1 and outlet pressure at test port TP2. Use these readings to examine:
Decaying set pressure (low pressure at port TP2). If pressure at the outlet port drops below the desired set pressure, check the pilot head spool and seat for excessive wear which may permit increased drain flow. Excessive drain flow through this section of the valve reduced the pressure needed in the chamber above the amin spool to increase valve pressure drop and limit operating pressure in the branch circuit.
Valve will not hold reduced-pressure setting (high pressure reading at port TP2). If preset pressure exceeds desired values, check for a:
Valve cannot be adjusted to desired low-pressure setting (high pressure reading of port TP2). If the valve cannot be adjusted to its desired pressure setting after the adjustment knob has been turned to its closed or nearly closed position, check for:
Fluctuating pressure or no pressure at output port (zero pressure reading at port TP2). If there appears to be no fluid pressure at port TP2, check to establish if: