Too Long; Didn't Read
The piston that lets air in and out controls the valve of your seat height level is no longer working. Most desk-chair cylinders flop because of faulty seals. They become too defective to sustain the required pressure. Another issue of the sinking has to do with the adjusting valve. If it is fixed incorrectly, it fails to control airflow due to which air gets into the cylinder and makes the spring behave irregularly. It would help if you found them so that you can know how to fix them.