Floor Jack Troubleshooting

[sabinoerc]

I have an old floor jack from Harbor Freight. A Central Hydraulics Model 34271 if that helps.
It doesn't take a full pump as it get's higher. Smaller and smaller strokes w/ pressure. Longer and Longer to get it to max height.
I thought maybe oil was low. I removed the plug on ram and put some oil in, not much - pumped it a bit to see if I could see any air bubbles, but nothing.
It still did the same thing.
I removed the handle so I could push the pump piston up/down by hand to see how it feels.
I found that as the jack went higher and the ram extended further - the pump piston would **** back down when pulled up. If I held it up long enough - it would take a full "bite/pump".
I've not messed with hydraulic jacks before but assume this is something like a plugged vent? As the ram goes out - something has to come in.
Just sanity checking if I'm on the right track. Checking if this is a known failure mode with whoever knows hydraulic jacks.

The only thing I see which may be a vent is a "do not adjust" cap. There is also some screw/filler but I don't see any vent holes in it. The filler plug is on the ram body and is a rubber stopper type plug.

Thanks in advance for any education/advice/direction.
Ed

 

[Hiball]

I suspect the pump piston seal is weak and can’t overcome the vacuum as the oil is transferred from the reservoir to cylinder. I quick check would be to pull the fill plug on the reservoir and then operate the jack and see if the condition remedies itself.

 

[sabinoerc]

Thanks.
Yes, it pumps well/correctly when the rubber fill plug is removed. Put it back in and towards top of range it again can't get full stroke - deform the plug to let air hiss into reservoir and it can pump a full stroke.

I read up a bit on the hydraulic circuit and now understand the screw on pic above is for the check valves/balls and the "do not adjust" is like the safety/over pressure relief.

What I don't understand is why the reservoir isn't vented?
The seals on pump piston are good enough that when I pull it up by hand the vacuum in the reservoir will just **** it back down if I let go.
Better seals would seem to have the same problem? - the vacuum in reservoir would still **** it back?
I'm probably missing something, thanks for the help.

 

[ajchien]

I think you got the idea right. The pump seal is fighting against the reservoir vacuum, and the higher the jack goes the greater the vacuum in the reservoir. When a seal gets weak, it can’t overcome the vacuum. Having a vent instead of a rubber plug or screw solves the problem. I think one reason some jacks have plugs or screws in the fill hole instead of a vent is to prevent oil spillage during shipping.

 

[Miss the Pontiacs]