Floor jack bleeds hydraulic pressure when not under load

[Wood_Guy]

I came across your forum while searching for an answer to a particularly vexing issue I have come across with one of my old Lincoln floor jacks (93652).

This was purchased rebuilt about eight years ago. I just checked the cylinder internally and the seals on the ram/piston are all perfect. There is no problem with it holding a load, and in 6-8 hours it holds over 1200-1500 lbs without dropping even a 1/4".

The issue is that when the jack is not under any type of load it appears to bleed off pressure over a period of time. For example, if you are working on a suspension and are using it to lift up a control arm, over a period of an hour it might drop down on its own (very little weight). If you just jack it up to full height and leave it without it lifting anything, in several hours it might drop down an inch or so, and appears to continually bleed off pressure. I have two others, and this is the only one that does it.

Is this an issue with the release valve? Any thoughts would be appreciated.

 

[paulsomlo]

Lip seals work better under pressure - it forces the lips out, against the cylinder walls. I looked online at the seal kit for that jack; it appears that it's not "energized", which means there's no oring between the inner and outer portions, so, what your seeing doesn't surprise me. Google "loaded ucup", you'll see what I mean.

 

[btdobie]

I wonder if the check valve in the pump isn't seating until it has some back pressure.

 

[Wood_Guy]

Lip seals work better under pressure - it forces the lips out, against the cylinder walls. I looked online at the seal kit for that jack; it appears that it's not "energized", which means there's no oring between the inner and outer portions, so, what your seeing doesn't surprise me. Google "loaded ucup", you'll see what I mean.

Out of the three only one does it.

I wonder if the check valve in the pump isn't seating until it has some back pressure.

This is possible. I just changed the oil in the jack, so maybe it was disturbed during the process of turning it upside down.

 

[paulsomlo]

Out of the three only one does it.
This is possible. I just changed the oil in the jack, so maybe it was disturbed during the process of turning it upside down.

So, the three jacks are all the same model? Same seals? And the one that does it is the only one that's been rebuilt?

It's always done this, or just since you changed the oil?

 

[Wood_Guy]

So, the three jacks are all the same model? Same seals? And the one that does it is the only one that's been rebuilt?

It's always done this, or just since you changed the oil?

All three are the exact same model with the exact same seal types. They've all been rebuilt at varying stages.

I just noticed it started doing this recently when I had it as a cautionary support under a rear diff without any significant weight, and left for a couple hours. When I came back it had dropped down about 2 inches.

Did a load test on it of about 1,200 lbs and it didn't drop at all in eight hours.