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On an air system, if air is detected escaping from a 4-way valve exhaust while the cylinder is stopped, this air is either blowing by worn-out piston seals, or is leaking across the spool in the 4-way valve. These two leakage paths are shown in the figure to the right.

Most cylinders and valves have soft seals and should be leak-tight. However, those air valves having a metal-to-metal seal between spool and body may be expected to have a small amount of leakage.

If leakage is noted, it is more likely to be coming through the cylinder than across the valve spool, and the cylinder should be tested first.

Two Leak Paths
Cylinder Testing

Run the piston to one end of its stroke and leave it stalled in this position under pressure. Crack the fitting on the same end of the cylinder to check for fluid leakage.

After checking, tighten the fitting and run the piston to the opposite end of the barrel and repeat the test. Occasionally a cylinder will leak at one point in its stroke due to a scratch or dent in the barrel. Check suspected positions in mid-stroke 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 make le ak intermittently. This is usually caused by a soft packing or O-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. The open line from the valve should be plugged or capped since a slight back pressure in the tank return line 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 "leak-tight" soft seals may have a small bypass during new seal break-in or after the seals are well worn.

Diagram for Cylinder Testing
4-Way Valve Testing

For testing 4-way valves, either air or hydraulic, it is necessary to obtain access to the exhaust or tank return ports so that the amount of leakage can be observed. To make the test, disconnect both cylinder lines and plug these ports on the valve. Start up the system and shift the valve to one working position. Any flow out the exhaust or tank return line while the valve is under pressure is the amount of leakage. Repeat the test in all other working conditions of the valve.

Reference > Cylinder & Valve Testing

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