Sinking Office Chair

Two of my office chairs (one in front of the vertical mill, one in my office) have a frustrating problem: after a few minutes of sitting in the, the seat starts slowly sinking.  You can pull up on the height adjuster, and restore it to your preferred height, but it won't stay there forever.  One of these chairs I bought new, several years ago and it worked fine for several years.  The other I bought for $25 at a thrift store, and it was otherwise like new; all the leather was flawless.  I suspect the sinking problem was why it went to a thrift store.

I couldn't sleep so I decided to see if there were any easy DIY solutions.  There were many:

At the core of these fixes is putting in a spacer tube to prevent downward motion once the hydraulic cylinder gives up.  This makes the chair one height only (or higher), but no one else sits in these chairs, so who cares?

The second one requires less work.  I may have some acetal tubing in my spare scraps collection that will do the job.  (It is 14^o outside; the garage is probably not much higher.  I'll wait for morning.) The first requires some chair disassembly to put the tube around the hydraulic cylinder.  I will use aluminum tubing to do this fix.

This one shows how to replace the hydraulic cylinder making it as good as new:

Replacement cylinders are shockingly cheap.

UPDATE: I think the talk of surgery scared them (or the cold has shrunk down the leaking surfaces to an airtight fit).  Both chairs are now behaving themselves.