Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. Infact, the cylin­ der may stall under light loads or may not move at all. Often the loss of power is accompa­ nied by an increase in pump noise, especially as the pump tries to build up pressure.

Any major component, i.e., pump, relief valve, directional valve, or cylinder, could be at fault. In a sophisticated system other components could also be at fault, but this would require the services of an experienced technician.


By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area. Then, if necessary, each component in that area can be tested or replaced.

1. Pump Suction Strainer

Cavitation of the hydraulic pump is the most frequent mode of hydraulic system fail­ure. The most frequent source of cavitation is suction flow restrictions caused by dirt buildup on the suction strainer. This can happen on both new and older systems. It produces the symptoms just described: increased pump noise and loss of high pres­ sure and/or speed.

If the strainer is not located in the pump suction line, it will be found immersed below the oil level in the reservoir. Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump.

The suction strainer should be removed for inspection and should be cleaned before reinstallation. Wire mesh strainers can best be cleaned with an air hose, blowing from the inside out. They can also be washed in a solvent that is compatible with the reser­ voir fluid. Kerosene may be used for strainers operating in petroleum-based hydraulic oil. Do not use gasoline or other explosive or flammable solvents. The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced.

When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints. There must be no air leaks in the suction line. Check reservoir oil level to be sure it covers the top of the strainer by at least 3 inches at minimum oil level, which is with all cylinders extended. If it does not cover to this depth, there is danger of a vortex forming that may allow air to enter the system when the pump is running.

2. Pump and Relief Valve

If cleaning the pump suction does not correct the trouble, isolate the pump and relief valve from the rest of the circuit by disconnecting so that only the pump, relief valve, and pressure gauge remain in the pump circuit. Cap or plug both ends of the plumbing that has been disconnected. The pump is now deadheaded into the relief valve. Start the pump and watch for pressure buildup on the gauge while tightening the adjust­ ment on the relief valve. If full pressure can be developed, obviously the pump and relief valves are operating correctly, and the trouble is to be found further down the line. If full pressure cannot be developed in this test, continue with step 3.

3. Pump or Relief Valve

If high pressure cannot be obtained in step 2 by running the pump against the relief valve, further testing must be conducted to see whether the fault lies in the pump or in the relief valve. Proceed as follows.

If possible, disconnect the reservoir return line from the relief valve. Attach a short length of hose to the relief valve outlet. Hold the open end of this hose over the reser­ voir filler opening so that the rate of oil flow can be observed. Start the pump and runt the relief valve adjustment up and down while observing the flow through the hose. If the pump is bad, there will probably be a full stream of oil when the relief valve adjustment is backed off, but this flow will diminish or stop as the adjustment is increased. If a flowmeter is available, the flow can be measured and compared with the pump catalog rating.

If a flowmeter is not available, the rate of flow on small pumps can be measured by dis­ charging the hose into a bucket while timing with the sweep hand on a watch. For exam­ ple. if a volume of 10 gallons is collected in 15 seconds, the pumping rate is 40 gpm.

If the gauge pressure does not rise above a low value, say 100 psi, and if the volume of flow does not substantially decrease as the relief valve adjustment is tightened, the relief valve is probably at fault and should be cleaned or replaced as instructed in step 5.


If a full stream of oil is not obtained in step 3, or if the stream diminishes as the relief valve adjustment is tightened, the pump is probably at fault. Assuming that the suc­ tion has already been cleaned and the inlet plumbing has been examined for air leaks, as in step 1, the oil is slipping across the pumping elements inside the pump. This can mean a worn-out pump, or too high an oil temperature. High slippage in the pump will cause the pump to run considerably hotter than the oil reservoir temperature. In normal operation, with a good pump, the pump case will probably run about 200″F above the reservoir temperature. If the temperature difference is greater than this, excess slippage, caused by wear, may be the cause.

Check also for slipping belts, a sheared shaft pin or key, a broken shaft, a broken cou­ pling, or a loosened set screw.

5. Relief Valve

If the test for step 3 has indicated the trouble to be in the relief valve, the quickest remedy is to replace the valve with another one known to be good. The faulty valve may later be disassembled for inspection and cleaning. Pilot-operated relief valves have small orifices that may be blocked with accumulations of dirt. Blow out all passages with an air hose and run a small wire through orifices. Check also for free movement of the spool. In a relief valve with pipe thread connections in the body, the spool may bind if pipe fittings are over­ tightened. If possible, test the spool for binding before unscrewing threaded connections from the body, or screw in fittings tightly during inspection of the valve.

6. Cylinders

If the pump will deliver full pressure when operating across the relief valve in step 2, both the pump and relief valve can be considered good. If so, the trouble must be further downstream. The cylinder should be tested first for worn-out or defective packing.

The easiest method for testing a hydraulic cylinder is to run the piston to one end of its stroke and leave it stalled in this position under full pressure. Crack the fitting on the same end of the cylinder to check for fluid leakage. After checking, tighten the fittings and run the piston to the opposite end of its stroke and repeat the test. Occasionally a cylinder will leak at one point in its stroke because of a scratch or dent in the barrel. Check suspected positions in rnidstroke by installing a positive stop at the suspected position and run the piston rod against it for testing. Once in a great while a piston seal may leak intermittently. This is usually caused by a soft packing or 0-ring moving slightly or rolling into different positions on the piston, and is more likely to happen on cylinders of large bore.

When making this test on hydraulic cylinders, the line should be completely removed from a cylinder port during the test and an open line from the valve should be plugged or capped, since a slight back pressure in the tank return would spill oil from the line if not plugged. Pistons with metal ring seals can be expected to have a small amount of leakage across the rings, and even those leak-tight soft seals may have a small bypass during break-in of new seals or after the seals are well worn.

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