Sinking feelings

As I sit at my computer I just can't sit completely still. I fidget. I change posture. I don't like it, but I gradually sink lower and lower, little by little descending into an uncomfortable slump. The cause is clear. The gas-lift mechanism on this office chair is leaky. It's been that way for a while (many months, actually). If I pull the release trigger to raise it before I sit, then do my best imitation of a corpse at the keyboard, it holds its gas nicely... if only I could sit still.
Today was the day to remedy the droop. No more would I feel deflated as I typed. The shop where we bought the chairs three years ago stock replacements.
Step 1: Disassemble the chair. It sounded easy, but the lift mechanism was jammed into the chair base and extremely reluctant to come out. The encouragement it needed was provided by my hammer.
Step 2: Buy replacement mechanism. This was easy, but the electronic gadgets section of the shop did its level best to distract me from my quest.
Step 3: Reassemble the chair. Simplicity itself.
Step 4: Test the new, firm, good-as-new chair. What? It droops! Eeeeeek! I'm sinking!
The new one was considerably worse than its slightly flacid predecessor. Guess what I shall be doing tomorrow.