How do you add hydraulic fluid to a hydraulic cylinder jack?

How to add hydraulic jack oil to an hydraulic jack (floor jack or bottle jack).

First, don't open the Check Valves!

The most common mistake with a floor jack is to open one of the check valves (or their covers) instead of actually finding the fluid reservoir plug. Be sure you have found the filler hole (which can be very tricky to find on an oily old jack -- it's probably a small rubber plug that might even be painted the same color as the rest of your jack, hiding on the housing of the main hydraulic ram, NOT on the base. The screw/bolt heads on the base are the check valves. If you open a check valve, the springs and check balls might come out under pressure, and your jack won't ever work again unless you can find them and put them back in the right places

If your jack needs oil, even after a couple of years, it probably also needs new seals. Adding oil might let you use it a while longer, but you should probably replace the seals (or if it's a cheap Chinese/Taiwanese jack, just get a new jack). American- or Japanese-made jacks are probably worth resealing. Please always use jack stands so when the seals really blow out you don't end up crushed.

You're probably going to just put jack oil in anyway (or maybe you did replace the seals and now you need to put jack oil in), so here's how to do it:

  1. Use Hydraulic Jack Oil (available at auto parts stores). Do not use brake fluid, which will turn the seals into useless cheese.
  2. Be sure you have found the oil reservoir hole, not one of the check valves. Do all the following work with the jack rightside up (and don't store the jack upside down or on its side).
  3. Clean the plug and jack body around the hole. Any debris in the oil can easily cause damage later.
  4. Release the valve/lower the jack (turn the jack handle counterclockwise).
  5. Unscrew or pry out the plug as appropriate. Fill the reservoir. A tiny funnel will probably make this a LOT easier).
    1. Close the valve and raise the jack slowly enough to not blast oil back out the hole. The reservoir level will go down, which is normal, and you shouldn't add oil now.
    2. Slowly lower the jack (again, just don't blast a bunch of oil back out the hole). Air bubbles may come out, causing a few drips of oil to come out too, in which case you should add oil until the reservoir is full, then go back to the previous step until you can lower the jack slowly without a bunch of bubbling.
  6. Bleed the air out of the system:
  7. Re-plug the filler hole, test out the jack, and you're done, although you should still replace the leaking seals if you didn't already.

Remember to always work on hard, level ground, use jack stands, chock the wheels, and set the parking brake.