If you can pump it up, but it drops down quickly, you may have air in the system that can be purged out when you add more jack oil. Chinese hydraulic actuated tools are notorious for developing leaks. If you remove the jack from the crane, you should be able to put it on its side and find and remove the =93air bleed=94 plug. If you pump the jack part way, you can more easily add jack oil in this hole. Cycle the ram up and down, and make sure that you have a pan or rags to catch any overflow. Jack oil can be obtained from your local auto parts store. Maybe they=92ll have this version:
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I=92ll use automatic transmission fluid as a substitute.)
Lots of people are stuck with cheap floor jacks that they feel that they should be able to repair, but just can=92t find parts. I=92ve taken apart floor jacks and re-assembled them with o-rings or homemade seals with middling success. If you disassemble yours, make sure that you keep track of the parts and don=92t loose the ball bearing or spring for the check valve. I=92ve spent hours hunting one down after it rolled under the workbench.
It=92s interesting to see how they=92re assembled anyways. I found pictures of someone rebuilding a floor jack here:
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one shows what looks like cup seals and leather backing. The seals that are on pumps vary, but this gives a general idea of what you might expect if you start disassembly. Some real old-time jacks used leather, rope or string packing that could be compressed with a sleeve and gland nut similar to what was found on older gate valves or water pumps:
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I=92ve still not figured out why most Chinese jacks fail. I=92m not sure of the materials used in their seals. (Nitrile (Buna-N) should be the best choice.) Could be poor fit up and excessive clearances cause the seals to extrude and fail prematurely. I=92ve cleaned out machine chips that caused the balls on the check valves to leak. Could be a bad seat where the ball fits in the check valve. In that case, the pump doesn=92t hold its position under load. Bad pump design might pull the piston out too much causing it to wobble and prematurely stretch and wear the seal.
Some people have tried to make a go of supplying repair parts for imports, but it doesn=92t seem profitable:
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Good primers on jacks:
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,
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Background on hydraulic jacks and repairable brands:
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,
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Hydraulic seal types:
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seal materials and failure modes (industrial):
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If I were to re-build another jack, I might get some hydraulic seals or rod seals from McMaster Carr or machine a new pump with a packing gland.
Different company from the one quoted above. (These guys claim to sell parts that will fit the imports, but I haven=92t used them, can=92t vouch for them, and it might be a pricey gamble for what you are trying to fix):
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Lots of people say to save time and money by buying only good quality products. They=92re probably right, but I still like figuring out what goes wrong and why.