Tools, Sharpening and Power Machinery - Hydraulic Jack Help

My hydraulic floor jack has a lift of about 18" which it will lift if there is very little weight. Once I try to lift something heavy it will only lift about 4", but it will hold forever at that height. Because it will hold I'm thinking it isn't seals or check balls so I was thinking I might have air in the system but I really know nothing about these things. Is an air lock a possibility & if so how do I fix it, or does this sound like a different problem? Also, I know they recommend using only hydraulic jack oil, but I've been using Esso Mineralube 30 hydraulic oil in another jack with no problem. Is this stuff okay to use? Thanks for any help on this.

Arn

How does the handle "feel" when you continue to pump the jack? Also, have you checked the oil level?

My jack does funny things like that also. The fluid may need to be topped off. After topping it off, it may still act funny, so you may have to pump the jack unloaded through its whole travel range a couple times. This seems works for me.

When I continue to pump the jack after it maxes out it feels like there is no resistance on the handle. I had the plunger & check ball out a few months ago & never had to replace the couple of ounces of fluid that were lost, that is, when I went to top it off the reservoir appeared full. Funny thing though, all of a sudden it started to work fine. That was yesterday & it still works today. Would air in the system perhaps cause something like this?

Arn,
Air can do some strange things. If you loosen the fill plug a little bit (just enough to let air get through) and cycle the jack through it's range, trapped air should be worked back out of the piston into the reservoir. Let it down slow enough not to make a mess. After a few cycles, check the level of the oil and add whatever is needed to bring it to full.

Yes it sounds like a good bleeding is in order

A good bleeding you say. Reminds me of an old Monty Python skit. I followed the bleeding instructions, added a few ounces of oil & it feels much firmer now when I pump it. Thanks for the help gang.

Arn