To repair a hydraulic floor jack, clean it, remove the plug on the cylinder, drain the used hydraulic fluid, and replace it with fresh fluid. Put the rubber plug into place, and test to see if it works.
You must clean a hydraulic floor jack before any cylinders are opened. Use rags to wipe away debris, and then use a flat-head screwdriver to pry off the rubber plug on top of the cylinder. Check the plug for damage. If there is any, replace it; if not, wipe it clean.
Place a bucket under the jack, and move it so the fluid can drain from the open plug on the top of the cylinder. Once the cylinder is empty, inspect the inside of it. If any debris is left, use compressed air to remove it.
Once the cylinder is clean, you can replace the fluid. While the amount needed varies by the size of the jack, the cylinder must be completely full. If no hydraulic fluid is available and the jack needs to be used immediately, you can substitute 5W-30 motor oil. However, drain and replace the oil with hydraulic fluid as soon as possible.
Clean the rubber plug once more before placing it back into the hole on the top of the cylinder. Make sure the plug is flat upon insertion; any gaps make the cylinder vulnerable to air leaks.