If your car lift or shop press seems to lose height on its own overnight, or a cylinder feels spongy under load even though there’s no puddle on the floor, you’re probably dealing with a hydraulic cylinder leaking internally. It’s one of the trickiest hydraulic problems to diagnose because there’s no visible fluid trail to follow — the oil is bypassing inside the cylinder itself instead of escaping outside the housing. We’re Auto Lift Services, an Iowa-based installer and parts supplier out of Ames, and we get calls about this almost every week from shops all over the state. Here’s what actually causes it and what we recommend doing about it.
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What “Leaking Internally” Actually Means
Every hydraulic cylinder relies on a piston seal that separates the fluid on one side of the piston from the other. When that seal wears out, cracks, or hardens with age, fluid can slip past it under pressure instead of doing useful work. That’s a hydraulic cylinder leaking internally — the oil never leaves the cylinder body, it just moves from one chamber to the other, which lets the piston creep backward under load. You won’t see a drip on the shop floor, but you will see the symptoms: a lift that settles a few inches after you walk away, a press that loses force partway through a job, or a cylinder that feels weak even though the pump sounds fine.
This is different from an external leak, where fluid actually escapes past the rod seal or a fitting and shows up as a wet spot on the cylinder barrel or the floor beneath it. External leaks are usually easier to spot and often cheaper to fix. An internal leak is quieter and more deceptive, which is exactly why so many shop owners run on a slowly failing cylinder for months before they realize what’s actually going on. We’ve walked into more than one Iowa shop where the operator assumed the pump was weak, when the real problem was a cylinder leaking internally the whole time.
Common Causes We See in the Field
The single most common cause is simple wear. Piston seals are wear items, and after years of daily cycling — especially in a busy shop running two or three lifts all day — the seal material thins out and stops sealing fully. Contaminated hydraulic fluid speeds this up dramatically; dirt, metal shavings, or moisture in the fluid act like sandpaper on the seal and the bore surface every time the piston moves.
Heat is another big factor. Cylinders that run hot, either from being undersized for the load or from a pump working harder than it should, cook the seal material and cause it to lose its elasticity. We also see scored or pitted cylinder bores, usually from rust that formed during a period when the equipment sat unused, or from that same contaminated fluid we mentioned. Once the bore itself is damaged, even a brand-new seal won’t hold pressure, because the seal needs a smooth surface to ride against. Overloading a cylinder beyond its rated capacity, even occasionally, can also deform the piston or seal grooves enough to start a slow internal bypass that gets worse every month.
How to Confirm It’s an Internal Leak
The classic field test is a drift test. Raise the lift or extend the cylinder fully with no load, mark the position, and walk away for fifteen to twenty minutes. If it settles noticeably with no visible fluid anywhere on the cylinder or fittings, that’s a strong sign of a hydraulic cylinder leaking internally rather than an external leak or a check-valve problem in the power unit.
It helps to isolate variables before you condemn the cylinder. Check that the lowering valve or check valve in the power unit is seating properly, since a bad check valve can mimic the same drift symptom and is a much cheaper fix. Inspect every fitting and hose along the way for weeping fluid, because sometimes what looks internal is actually a slow external leak at a fitting that’s hard to see from most angles. If everything upstream checks out clean and dry and the cylinder still loses height under a static load, the seal or bore inside that cylinder is almost certainly the problem.
Repair or Replace: How We Make the Call
Not every internal leak means a new cylinder. If the bore is still smooth and the cylinder body isn’t pitted or scored, a seal kit rebuild often solves it completely and costs a fraction of a new unit. We stock seal kits for Rotary, Challenger, BendPak, and Atlas cylinders and can usually get a shop back up and running within days rather than weeks.
Where the bore is damaged, rusted, or the cylinder has been overloaded to the point of deformation, rebuilding is a temporary fix at best — the new seal will wear against the same bad surface and fail again, often faster the second time. In those cases we recommend a full cylinder replacement rather than throwing good parts at a bad bore. We walk every customer through both options honestly, including realistic downtime estimates, before they commit either way.
Why You Shouldn’t Ignore a Slow Internal Leak
A cylinder that’s leaking internally doesn’t fail all at once, and that gradual decline is exactly what makes it dangerous. As the internal bypass worsens, the lift or press loses more and more holding capacity under load, which means a vehicle or heavy component can settle further and faster than the operator expects. We’ve seen technicians caught underneath a lift that dropped several inches in the time it took to swap a part, simply because nobody had traced the slow drift back to its source.
Beyond safety, a hydraulic cylinder leaking internally also runs your pump harder than it should have to work, since the pump is constantly fighting fluid bypass to maintain pressure. That extra strain shortens pump life and can mask other developing problems in the hydraulic system. Catching the leak early, while it’s still a seal issue and not a bore issue, is almost always the cheaper and safer outcome. If you’re already noticing drift on your lift, our guide on lift hydraulic cylinder leaking symptoms covers the broader troubleshooting picture.
Preventing Internal Leaks Before They Start
Fluid quality is the single biggest lever you control. Using the hydraulic fluid specified by the manufacturer, and changing it on the recommended schedule instead of running it until it looks bad, keeps contaminants from grinding away at seals and bores. A cheap in-line filter or simply keeping reservoir caps sealed against shop dust goes a long way over a cylinder’s lifetime.
Regular visual inspection matters too. Catching a cylinder that’s beginning to lose a small amount of height, before it becomes a full internal leak, gives you the option of a low-cost seal kit rather than a full replacement down the road. We also recommend never exceeding rated capacity, even briefly, and having a technician check cylinders annually as part of routine lift service. If you’ve already replaced a cylinder due to an internal leak before, our page on hydraulic cylinder leaking repair options walks through what a proper rebuild versus replacement job actually involves.
When to Call In a Professional
Some internal leaks are simple enough to diagnose and fix in-house if you’re comfortable pulling a cylinder and installing a seal kit correctly. But bore honing, proper seal seating, and correctly bleeding air out of the system afterward all take experience, and a botched rebuild can fail again within weeks. If you’re not confident isolating the problem yourself, or if the equipment supports vehicles and people underneath it, it’s worth having someone experienced take a look rather than guessing.
Our crew handles this diagnosis routinely across Iowa, and we can usually tell over the phone, based on your symptoms, whether you’re looking at a quick seal kit or a full cylinder swap. If you’ve read this far because your own equipment is showing the classic signs of a cylinder leaking internally, don’t wait for it to get worse — reach out and we’ll help you get it sorted before it becomes a safety issue.
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