A customer called about a hydraulic cylinder that was “leaking badly.” The seal had been replaced twice. Each time, the cylinder ran for about two weeks and started weeping again. The customer was frustrated. They bought OEM seal kits. They replaced the wipers. They cleaned the rod. Nothing held.
When we arrived, the cylinder was on a clamping application, 50 mm bore, 200 mm stroke, pushing at 140 bar. The rod looked wet. We wiped it dry and watched for 10 minutes. The rod stayed dry. But the fitting at the base of the cylinder — the bleed plug — was damp. We touched it. It was hydraulic oil. The “leaking seal” was actually a seepage at the bleed plug, running down the rod and looking like a rod seal failure.
The bleed plug was a flat-head set screw with no seal. It had loosened from vibration. The oil was weeping past the threads. It ran down the cylinder barrel, across the end cap, and onto the rod. The wiper was doing its job — it was wiping oil that wasn’t from the seal. The customer replaced the seal twice because the rod looked wet. The wet rod was the symptom, not the cause.
We put a copper washer on the bleed plug and torqued it to 15 N·m. The weeping stopped. The cylinder ran for two years without a seal replacement. The seal kits they had bought sat on the shelf. We didn’t charge them for the service call — it was a 10-minute fix. They were embarrassed but relieved.
This is harder than it sounds. When a cylinder looks like it’s leaking, the first instinct is to rebuild the seal. But the seal is the most visible part. The fitting, the line, the port — those are hidden. Next time a cylinder leaks, wipe it dry and trace the oil path upward. The leak is usually at the highest point in the oil path, not the lowest. The oil runs down. The wet spot is where it ends up, not where it starts.
