The majority of office chairs allow you to adjust their height through the use of a pneumatic cylinder. The pressure of the gas in the cylinder pushes against your weight as you sit and if you release some of the pressure it will drop farther down as you sit, allowing you to lower the chair. Removing your weight from the chair allows the cylinder to fill up with gas again which gives more pressure and allows the seat to be raised higher. The cylinder is a sealed system with a pressurised area and a reservoir area, any leak will decrease the overall height ability of the chair and the cylinder cannot be refilled. If the leak allows the gas to escape completely then the whole cylinder will need to be replaced.
Flip the chair over so the wheels are facing up. Pry up the clip in the centre of the base with the flathead screwdriver and pull it off with the pliers. Remove the washer under the clip and lift the base off of the cylinder.
Slide the plastic housing off of the cylinder. Spray the soapy water over the cylinder in a light, even coat. Press down on the bottom of the cylinder and look for bubbles forming due to the leak. Note the location of the leak and wipe off the soapy water.
Mix a small amount of the metal epoxy components according to the manufacturers instructions. Roll it into a ball and then flatten the ball into a circle. Place the circle over the leak on the cylinder and press down the edges. Wait for it to cure according to the manufacturers specifications. It usually runs between four and 24 hours.
Tape the leak-area with duct tape all the way around the circumference of the cylinder. Replace the plastic housing then replace the base and reinstall the washer and retaining clip. Set the chair upright and test the pressure retention. If it is still losing pressure, the seals have likely failed and the entire cylinder will need to be replaced.