Hydraulic troubleshooting

Would you pass this simple hydraulics test?

The cylinder in the circuit shown below is drifting. In an effort to isolate the problem, the technician has installed ball valves 1 and 2. When both these ball valves are closed, the cylinder stops drifting. This proves to the technician that the cylinder's piston seal is not leaking and therefore he should replace the directional control valve.

hydraulic schematic

Is the technician right or wrong?

This conclusion assumes if the piston seal was leaking, the piston rod would continue to drift with the ball valves closed. But with ball valves 1 and 2 closed, the cylinder's piston seal could be missing completely and the cylinder would still NOT drift. So it is wrong to conclude the piston seal is not leaking.

So the technician is WRONG.

To understand why, watch this 8-minute video


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