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The pumping piston/cylinder is leaking lots of hydraulic fluid up the shaft on some pumping strokes, and is fine on other strokes.

The jack does not leak from anywhere else, it works to jack things up, and it does not seem to sink noticeably on its own.

The tip of the piston's circumference has uneven beveled wear. The piston and cylinder have some surface rust. I don't know how significant these observations are.

My first thought was to replace the o-ring, but it does not look damaged to my eyes, though I might still try to find a replacement if it's worth a shot.

Thoughts on what the issue might be and how to repair it if possible?

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Is it repairable?

Most likely yes. I don't see that there is an issue with the black o-ring, but if you'll notice, there's a second ring around that shaft, which is most likely nylon. I can't tell for sure, but it looks as though there might be an issue there. To fix your issue, you'd most likely need to replace both. You'd also need to check the barrel of the pump (what this rod goes into) to make sure it is round and there's no gouging or deformity in there.

The bigger unasked question is, is it worth your time/money to do so? Answer there is, probably not. You can get decent jacks as a replacement fairly cheap. I'd consider this jack a throw away item and be done with it. But that's up to you.

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  • The plastic/nylon ring looks "broken" due to the diagonal split it has for getting it on the piston/shaft. It doesn't look broken or deformed to me. Clicking the images of the barrel (or cylinder as I called it) will enlarge them. You can see some wear patterns, but I don't know if it's problematic. True, repairs often aren't worth it economically, but.. a DIYer doesn't give up that easily. I'm still trying to find out what the issue is and how accessible a solution is to me.
    – adatum
    Commented Jun 10, 2021 at 18:57
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    High quality, brand name floor jacks are always worth a rebuild. The real question is can you get a rebuild kit for that make/model jack. High dollar, high quality jacks, kits are available. No name cheap jacks, not so much. What you really don't want to do is chase individual O-rings and nylon washers on a hope and a prayer. Been there done that and it always failed. There is always one more part you need that is just not available.
    – zipzit
    Commented Jun 10, 2021 at 23:32
  • @zipzit I called MotoMaster customer service and they don't have a rebuild kit for this model, as it is discontinued. They pointed me to their supplier, Alltrade Tools, but that company doesn't seem to answer their phones. I chased around for the individual o-ring at auto parts stores and it's not easy finding oil resistant ones (and often rarer to find clerks who know the difference). Unfortunately they're usually sold in large sets, not individually. Another thought: could flushing and refilling the hydraulic fluid help? Maybe dirt blocking passageways and forcing oil past the o-ring?
    – adatum
    Commented Jun 12, 2021 at 0:59
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    @adatum - If it works at all, most likely that's not the problem or at least something which will fix anything.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jun 12, 2021 at 1:01
  • Yippee. It’s time for a new jack! Save up your pennies, buy a high quality name brand jack for which a rebuild kit will be available 20 years from now! If you really want to go fancy get an aluminum jack with two piece handle that you can easily lift and throw in your trunk for an on the road repair when a friend in need calls.
    – zipzit
    Commented Jun 12, 2021 at 14:39

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