How to disassemble or rebuild small hydraulic jack from kaolijack's blog

How do Hydraulic Jack come apart?

Anyone know a web page that tells how to disassemble or rebuild small hydraulic jacks? (That's a non-linux related question, isn't it? )

I have two hydraulic "bottle jacks" that don't work. One is from a Sears 3.5 ton floor jack. It looks to be about a 12 Ton jack and its only marking is "MVP". A valve in it must leak because a load that is jacked up will slowly descend. The other jack is a 12 ton jack from a shop press. It will jack things up, but when the valve is released, it does not descend unless there is a very large load on it. Although I'm not promising to repair these bottle jacks, I would like to know how to take them apart. For example, does the cylinder unscrew from the metal base?

I got around to fooling with the red 12 ton "Made in China" jack that used to be in my shop press. I popped out the fill button and emptied out the oil. On the top of the jack is a big hex nut about 2 1/8 inches across. I have no socket that big. I purchased a large wrench. I have no idea what this wrench is for. It isn't really an antique. It has a 1" diameter round, solid steel shaft with slight bends in it. One one end is a box wrench for 2 inch nuts and on the other end is a box wrench for 2 1/4 inch nuts. The overall lenth of this thing is about 51 inches and the box wrenches are have very thick sides. The handle has a slight bend near each end. It's very embarassing for a tool collector to actually use a tool for something, but the larger wrench more or less fit the nut on the jack. I put the jack on a heavy wooden work table and used deck screws and the power screw gun to clamp the base of the jack to the table with scraps of 2x6 lumber. Then I took the wrench and had a go at removing the nut. It came off quite easily - relative to the size of wrench being used. The nut and the cast part of the top that it is a part of come off together with the piston. The only seal at the top is a thin plastic ring about the size of a piston ring. The seal at the bottom of the piston is a tough plastic or rubber washer. I don't understand how it is held on the piston yet. It appears as if the washer might be pressed on the end of the piston. I noticed the seam where the outer casing of the jack meets the base. The paint had cracked there along a line, so I thought the outer casing might also come off. I got a (modern) pipe wrench about 20 inches long and gripped the casing. It also twisted off easily. It wasn't threaded. Apparently the pressure of the nut at the top holds it to the base.

The inside of the Hydraulic Bottle Jack is clean and not rusted. I can see that there is alot of resistance between the piston and the cylinder walls when I push it down by hand. The problem with this jack is that it did not go down easily when the valve was opened. I can't see much of the valves even with this much of the jack taken apart. The link on jacks warned against unscrewing the valve, but I think this is the only thing left to do. Either the valve must be bad ( it has a rubbery feel to it as you turn it) or the seal on the bottom of the piston is just mis-designed.