Why is my hydraulic pump leaking from the shaft?

A shaft seal rarely fails on its own. The usual causes are case pressure pushing the lip off the shaft, a groove worn into the shaft seal track, bearing wear letting the shaft move, coupling misalignment or side load, and the wrong seal material for the fluid or temperature.

Why does my new shaft seal keep leaking?

Almost always because the underlying cause was not fixed. Check case drain pressure first, since a shaft seal is rated for only a few psi and a restricted or wrongly plumbed drain line will blow out any seal you fit. Then check the shaft for a wear groove.

What should hydraulic pump case drain pressure be?

Low, typically single-figure psi on most units, because the case should drain freely back to tank. Higher readings indicate an undersized, kinked or wrongly plumbed drain line, and that will push the shaft seal out.

Can I replace a hydraulic pump shaft seal without removing the pump?

Sometimes, depending on the design and access, but it is rarely the best call. You cannot inspect the shaft track, bearings or case drain properly in place, and those are the things that caused the leak.

Which shaft seal material should I use?

Nitrile for mineral oil at normal temperatures, Viton for high temperature or aggressive fluids, and EPDM for phosphate ester and water glycol. Never fit EPDM in mineral oil or nitrile in phosphate ester.

Hydraulic Pump Leaking From the Shaft? Read This First

by RestoPower September 9, 2026

A shaft seal is a thin rubber lip riding on a spinning shaft. It is the lowest-pressure part of the whole pump and the one people replace most often, usually more than once, because a shaft seal rarely fails on its own.

Something caused it. Pressure it was never meant to see, a shaft surface that has worn a groove, a bearing letting the shaft move, or the wrong material for the fluid. Fit a new seal over any of those and you will be back under the machine inside a month.

The one measurement that decides everything: case drain flow, and case drain pressure. A shaft seal on most piston pumps is rated for only a few psi. If the case is pressurizing, the seal is being blown out, and no seal you fit will survive. Check that before you order anything.

Is it actually the shaft seal?

Oil tracks. Find where it starts rather than where it drips.

Where the oil appears	Usually
Slinging outward in a ring, wet all round the shaft, drive coupling oily	Shaft seal
Running down one face from a joint line	Cover, end cap or port plate gasket
Weeping at a fitting or hose end	Fitting, not the pump
Wet at the compensator or control	Control O-rings
Oil in the bellhousing on an engine-mounted pump	Shaft seal, or the case drain routed wrong

Clean it thoroughly, run it briefly, and look again. Guessing from a dirty pump is how the wrong kit gets ordered.

The five real causes

1. Case pressure

This is the big one on piston pumps and the reason most repeat leaks happen. The pump case is meant to drain freely back to tank at close to zero pressure. If the drain line is undersized, kinked, plumbed into a pressurized return, shared with another unit, or run up and over with no siphon break, case pressure rises and pushes the seal lip off the shaft.

Check: tee a low-range gauge into the case drain and read it running. It should be low, single-figure psi on most units. If it is not, fixing the plumbing is the repair. The seal is just the messenger.

2. A worn shaft

A seal lip running in one spot for years polishes and then cuts a groove into the shaft. Drop a new seal into the same position and it sits in that groove and leaks immediately.

Check: run a fingernail along the seal track. If it catches, the shaft is grooved. Options are a speedi-sleeve style repair sleeve, positioning the new seal slightly deeper so the lip runs on unworn surface, or replacing the shaft. On splined drive shafts, check the splines for wear at the same time.

3. Bearing wear letting the shaft move

The seal can only tolerate so much shaft movement. Once the support bearing wears, the shaft orbits, the lip cannot follow it, and it leaks. This one is worth catching because the same wear is about to damage far more than a seal.

Check: with the pump stopped, grip the shaft and try to lift and rock it. Noticeable play means bearings, and that is a teardown, not a seal.

4. Coupling and alignment

A misaligned coupling, a badly worn spider, or a driveline pulling the shaft sideways loads the seal and the bearing continuously. So does an overtightened belt on a pulley-driven pump.

Check: alignment at the coupling, the condition of the element, and whether anything is applying side load the pump was not specified for.

5. Wrong seal material, or heat

Standard nitrile is fine for mineral oil at sensible temperatures. Put it in phosphate ester, water glycol, or a system that runs hot and it hardens, shrinks and takes a set. Then it leaks even though it looks intact.

Check: what fluid is in the system and what temperature it runs at. Viton for heat and aggressive fluids, EPDM for phosphate ester and water glycol. If your pump has been overheating, the seal is a symptom. See why is my hydraulic pump overheating.

Never fit an EPDM seal into mineral oil, or a nitrile seal into phosphate ester. The two kits can look identical in the box. On Denison pumps the seal class is a digit in the model code: 1 is Buna-N, 4 is EPDM, 5 is Viton. Check it against your fluid before ordering. Details in the Denison decoder.

Doing the job properly

Fix the cause first. Case pressure, shaft groove, bearing play or alignment. If you skip this the rest is wasted.
Inspect the shaft surface and decide between a sleeve, a repositioned seal, or a new shaft.
Match the material to the fluid and the operating temperature.
Protect the lip on assembly. Splines and keyways cut seal lips. Tape the keyway or use an installation cone, and lubricate the lip with clean system oil.
Press it square, to the right depth. Cocked seals leak from new. If you are moving the seal deeper to clear a groove, confirm there is clearance behind it.
Check the drain routing before you start it up, and make sure the case is filled with clean oil so the seal and bearings are not run dry.

While it is apart, do the whole kit. A shaft seal on its own is a false economy when the cover and port O-rings are the same age and the pump is already open. The labor is the expensive part.

Seal kits by brand

Vickers: piston pump kits, vane pump kits, motor kits, Viton kits
Rexroth, Denison, Parker, Sundstrand
Char-Lynn motor kits, Poclain motor parts

Not sure which kit your unit takes? The decoders will tell you: Char-Lynn, Denison, Parker, Sundstrand, Vickers.

Send the model code and we will send the right kit

Tell us the full code, what fluid is in the system, and whether this seal has been replaced before. That last answer usually tells us what actually went wrong. Most quotes go out the same business day.

Request a quote · (810) 882-1171 · Repair and rebuild service