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 “air bleed” 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’ll have this version: http://www.amazon.com/JACK-OIL-STOP-.../dp/B0002KKIBY
(Sometimes I’ll 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’t find parts. I’ve 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’t loose the ball bearing or spring for
the check valve. I’ve spent hours hunting one down after it rolled
under the workbench.

It’s interesting to see how they’re assembled anyways. I found
pictures of someone rebuilding a floor jack he
http://www.garagejournal.com/forum/s...t=18055&page=4
This 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:
http://www.diybob.com/diyPackingGland.htm
http://www.monarch-mclaren.com/leather.htm

I’ve still not figured out why most Chinese jacks fail. I’m 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’ve 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’t 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’t seem profitable: http://www.hyjacks.com/H32.HTM

Good primers on jacks: http://www.hyjacks.net/animation.htm ,
http://www.hyjacks.com/H7.HTM

Background on hydraulic jacks and repairable brands: http://www.hyjacks.com/H2.HTM
, http://www.jackxchange.com/

Hydraulic seal types:
http://www.sealing.com/hydraulic-seals/
Hydraulic seal materials and failure modes (industrial):
http://www.epm.com/faq.html

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’t used them, can’t
vouch for them, and it might be a pricey gamble for what you are
trying to fix):
http://www.hydjack.com/krk305.html

Lots of people say to save time and money by buying only good quality
products. They’re probably right, but I still like figuring out what
goes wrong and why.