Sorry, I should have specified: The sag is definitely caused by air getting into the oil chamber, and the air chamber on the other side of the IFP is the nearest source.

The lifting-by-the-saddle thing is a theory people have, but borne out by my own limited experience. After one rebuild it was flawless, but the problem came back a short while later after a trip – no gradual sag, just suddenly an inch. After a second rebuild I learned to not lift it by the seat, and the problem hasn’t reoccurred in six months.

There are other theories, such as cold temperatures causing seals to shrink, but I’ve had it function fine and not develop the problem after sub-zero rides.

Edit: Thanks for the PDF links. I’ve been given a knackered stealth reverb and have been building up to having a poke around it’s innards.