It is very easy to solve this problem with your office chair. You do not need to order any parts and you do not need to buy a new chair. All you need to do is to find a PVC pipe connector roughly about the size of your chair bottom vertical steel extension in the centre. Unscrew the bottom part of your chair with a screwdriver and then attach the PVC connector onto it, threading it through the centre vertical steel frame. Screw back the bottom part of your chair and now your chair will not sink down anymore. However, you won't be able to adjust your chair height anymore, even if you pump air into your chair by lifting the side lever. Below is a picture of the quick fix that i did to prevent the office chair from going down.