Short answer: a floor jack that sinks under load is most often carrying air in its hydraulic system, not a failed seal — and the fix is the manufacturer's bleed procedure, which takes about ten minutes and no parts. The three causes, in the order you should check them, are a release valve that is not fully closed, trapped air, and a blocked valve. Worn seals come fourth, and they are the only one of the four that means the jack is finished.
Match the symptom first
The manual groups these faults into a single troubleshooting table, because several symptoms share a cause. Find yours before you touch anything.
| What the jack does | Most likely cause | First thing to try |
|---|---|---|
| Lowers under load | Release valve not fully closed, or air in the system | Close the valve hard, then bleed |
| Pump stroke feels spongy | Air in the system | Bleed the system |
| Saddle will not lift all the way | Low on oil, or air in the system | Check the oil level, then bleed |
| Handle moves up when under load | Valve blocked and not closing fully | Flush the valves |
| Oil weeping from the filler plug | Overfilled, or the plug is not seated | Drop the level to spec and reseat the plug |
| Oil weeping from the ram itself | Worn seal | This is the one that ends the jack |
Notice how much of that table resolves to "bleed it". Air is compressible and hydraulic oil is not, so a pocket of air in the cylinder behaves exactly like a leak: the saddle rises, then settles, and the stroke feels soft. It is the single most common reason a jack appears to be dying and the cheapest to put right.
Cause one: the release valve is not fully closed
Check this before anything else, because it costs nothing and it is embarrassingly common. The manual's first remedy for both "lowers under load" and "will not lift" is "check that release valve is fully closed". The valve is operated by rotating the handle, and a handle that is a few degrees short of home leaves the circuit slightly open. Turn it clockwise until it stops, then try again with no load.
Cause two: air in the system, and how to get it out
The manual says to bleed before each use or if jack performance decreases — that is the manufacturer treating air as a routine condition rather than a defect, which tells you how normal it is. Jacks stored on their side, shipped a long way, or left unused for a season all pick it up.
1.Lower the saddle fully and remove the load
Nothing on this page is done with a car in the air. Get the jack down, clear and on a level floor.
2.Turn the handle counter-clockwise to open the release valve
Fully open, not partly. The circuit has to be able to vent back to the reservoir.
3.Remove the cover plate, then the oil fill plug
On the 70482 that is four screws holding the cover plate, and the fill plug underneath it. Your jack may put the plug somewhere else — check the parts diagram in your manual.
4.Top off the hydraulic fluid
The manual's wording is to add high-grade hydraulic fluid slowly until it reaches the specified level. See the oil section below for what qualifies and what will destroy the seals.
5.Apply pressure to the saddle and pump the handle quickly, several times
This is the step that actually does the work: pumping against the open fill hole drives trapped air back out of the cylinder instead of compressing it.
6.Re-check the fill hole, top off, refit the plug and cover
Expect the level to have dropped once the air left. Do not overfill — an overfilled jack weeps from the plug, which is a different fault with the same puddle.
7.Close the valve and test several times before lifting a load
The manual is explicit about testing for proper operation before attempting to lift a load. Run the full stroke, leave it up for a few minutes, and watch whether it holds height.
Cause three: a blocked valve
If the handle rises on its own while the jack is under load, the manual attributes it to valves that are blocked and not closing fully, and gives a flush procedure: lower the saddle, close the release valve securely, then manually lift the saddle several inches. That pulls fluid back through the valve seat in the opposite direction and can shift the debris that is holding it open. It is worth two or three attempts before you conclude the jack is beyond help.
What oil goes in a floor jack
The honest answer is the one your manual gives, because the specified grade genuinely varies between makers. Harbor Freight's wording for the 70482 is high-grade hydraulic fluid, purchased separately. What matters more than the exact viscosity number is what you must not put in:
The short version
- Buy fluid sold as hydraulic jack oil. It is cheap, it is stocked in every auto parts store, and it is formulated for jack seals.
- Never brake fluid. It is glycol-based and it attacks the seals a jack uses — this is the single most expensive substitution people make.
- Never engine oil or transmission fluid. The detergents and friction modifiers are wrong for the job and the viscosity is wrong for the pump.
- Never improvise with whatever is on the shelf. A jack holds a car over your body; the fluid is not the place to economize.
- Check your own manual for the grade and the fill level. Both vary, and overfilling causes its own leak.
- Change it on a schedule, not on failure. Harbor Freight specifies a full fluid change at least once every three years.
The same manual also asks for the grease fitting to be lubricated once a month and the jack to be wiped dry after each use, and gives a drain, refill and bleed sequence for the three-year fluid change. None of that is demanding, and a jack that has had it done does not usually develop the fault that brought you here.
When to stop repairing and replace it
Seal kits exist and they are inexpensive, but the labor is fiddly and the outcome is uncertain on a jack that has already been apart once. We would replace rather than repair in four situations: oil weeping from the ram body rather than the fill plug; a bent or visibly distorted lifting arm; a cracked weld anywhere on the chassis; or a jack that still sinks after a correct bleed and a fluid change. The last of those is the decisive one, because it means the fault is not air.
If you are replacing, best floor jacks is the ranked field, best floor jacks under $100 is the cheap end, and best floor jack brands covers which makers publish real specifications and which do not.
The order to work through
Close the release valve properly. Bleed the system following the sequence above. Flush the valves if the handle rises under load. Change the fluid if it has been three years or you do not know. Test with no load, twice. If it still sinks, buy a new jack — and in the meantime, keep the car on stands like you were already supposed to.