When your office or desk chair begins sinking after sitting on it the problem is caused by the valve on the cylinder not working properly and closing to keep the air inside the cylinder.
If the valve on the chair cylinder is bad then your only option is to get a replacement cylinder if the office chair is a higher priced office chair.
You can usually find the replacement cylinders from the manufacturer or sometimes on ebay.
If the office chair is a cheaper chair that is under $100.00 then I would just buy a new office chair instead.
But you can use a hose clamp on the cylinder to stop it from sinking down if you want to keep the chair and can't afford to buy a new office chair right now.