Whilst I had the power steering pump off the flagship yesterday, I checked the vac advance and found it to be dead. Decided to open it up for a repair, as I did with one from my 88 3.5 RR a several years ago. Haven't priced one of these things in years, but I recall the replacement cost easily justified a half-hour repair job last time.
The earlier unit had a screw holding the diaphragm onto the pushrod, so was a pretty easy repair. This 94 model unit is not so repair-friendly; the washer holding the diaphragm onto the pushrod is held in place with a spot weld. I brazed a small screw thread onto the end of the pushrod to make it repairable.
The earlier unit had a rubber diaphragm that had deformed and pulled away from the housing one one side, causing the failure. This later one, plus one from the 97 3.9 D1 that I've wrecked, has a synthetic diaphragm that has become brittle and pretty much disintegrated.
I replaced the diaphragm with a circle cut from the same bit of Clark Rubber sheet stock that has sat under my bench since the last vac advance repair.
The one that I repaired several years ago now lives on in my 1990RR and has done probably 150,000km since receiving a new diaphragm. This is a pretty simple and satisfying repair. I would recommend getting hold of an early vac unit to fix, as the threaded pushrod makes it very easy.