In construction machinery, the seals of hydraulic cylinders gradually wear out and fail during operation, leading to internal leakage (if you can’t spot external leakage, you must have had too much to drink at lunch). Internal leakage is not only harmful to normal machine performance but can also cause additional issues, and in severe cases, result in the hydraulic cylinder being scrapped.

Being able to correctly identify internal leakage in hydraulic cylinders is the main focus here.

Generally speaking, after about 5,000 working hours, an OEM hydraulic cylinder will begin to show signs of internal leakage under normal conditions. By 8,000–10,000 hours, internal leakage becomes severe. If the seals are not replaced in time, the cylinder may be scrapped.
I’m not selling you parts here, so I have no vested interest—what I’m saying is objective. If I had a profit motive, you could doubt my words; but without one, this information is reliable.

Diagnosis Methods (unchanged since the invention of hydraulic systems—long before I even drove a tractor):

1. Direct Test by Blocking the Circuit

   • Disconnect the rod-side hydraulic line of the cylinder under inspection.

   • Plug the hose end coming from the main control valve with a cap.

   • Start the machine, raise the boom until it leaves the ground (the higher, the better for testing), then stop the engine.

   • Observe the disconnected hydraulic line:

     • If hydraulic oil continuously flows or even spurts out, the cylinder seal has definitely failed.

     • If no leakage occurs, the seal is intact.

2. OEM Standard Sedimentation Rate Test

   • Use the manufacturer’s cylinder settlement (drift) standard.

   • If the cylinder settlement time is within 10% of the OEM standard, the seal is still serviceable.

3. Accumulator-Based Reverse Operation Test (requires the accumulator to be in good condition)

   • Start the machine and put the tested cylinder under load.

   • Shut off the engine, leave the key and hydraulic lock in the working position.

   • Push the control lever to the opposite end position and hold it there.

   • Observe:

     • If the cylinder settlement accelerates, the seal has failed.

     • If not, the issue is not seal-related.

   • This method also works on non-pilot-operated machines—proven effective countless times.