We have found in the past that a cylinder with a run-away load and no flow controls or counterbalance valves could draw a vacuum at the rod seal when the piston rod retracts faster than the pump flow could keep up with, drawing air into the rod end of the cylinder. We weren’t quite sure this fit the symptoms but stranger things have reared their heads before.

After convincing ourselves that the cylinder wasn’t the cause, we shifted our focus to the pump area. We checked the re-plumbing of the inlet and case drain lines and applied a heavy coating of #2 grease to each joint — one joint at a time — but could not find an air leak.

Thinking that something did not look right, we then inspected the shaft seal area. Having run a repair shop for 10 years, we had rebuilt many of these types of pumps. We soon realized the pump was either missing the normal outer shaft seal, or it had been installed backwards.

Unlike piston pumps, pressure compensated vane pumps are not built with tight tolerances. When they decompensate, vane pumps actually draw oil out of the intake line and the pump case housing. These pumps require a standard shaft seal with the lip facing toward the oil to prevent external leaking. But they also need a second shaft seal installed with the lip facing outward. That’s because pressure-compensated vane pumps tend to draw a vacuum in the case when they decompensate to full volume. Without the second seal, air is drawn past the oil seal in to the case and then into the system. Because the oil seal is designed to prevent flow in only one direction, two seals should be installed back-to-back. In this case, only a single seal had been installed.