To repair a sinking hydraulic chair, you need to replace the gas cylinder in the chair's base stand. You also need a plumber's pipe wrench, needle-nosed pliers, grease or Vaseline and WD-40. It is also recommended to wear eye protection, due to the possibility of nitrogen gas escaping.
Turn the chair on its side, and remove the base of the chair by using needle-nose pliers to gently pry off the clip on the bottom. After the clip is off, remove the washer, then slowly remove the chair's base. If any of the bearing parts remain inside after removal, they must be replaced in the order in which they were removed. Inspect the rubber bumper, bearing retainer and washer to make sure they are in good condition. If not, they need to be replaced.
Spray some WD-40 into the top of the gas cylinder, and let it sit for several minutes. After soaking, attach the pipe wrench to the top post of the gas cylinder, and attempt to loosen the cylinder, turning the wrench in either direction. It is important to wear eye protection at this point because the contents of the cylinder are under pressure and the wrench may cause a puncture and a nitrogen leak.
Once the old cylinder is off, hand insert the new one. Lightly grease the bearing with grease or Vaseline, and replace all the small parts in the order they came off. Finally, reinstall the base.