I have flown nothing much other than the FTDK for the last 500 hrs, so I for the first time in 35 yrs of flying I am pretty much in tune with this one aeroplane.
I normally use a 1200 rpm idle, and at that the suction warning light is not illuminated.
Last week, taxying in from a flight from "The Curry" I noticed the suction warning light illuminated at 1200 rpm and I thought "that's interesting". Had to increase rpm to 1300 to extinguish it.
"Its never done that before I thought to myself!"
Today, about half way into a flight from Townsville to Atherton while in VMC in top, the suction light illuminated, suction guage went down to zero.
Scratch one vac pump - and I only have one!
Fortunately I do have an electric standby AH and a couple of GPSs to steer by.
I have had some harsh words to say about Cairns ATC in the past, but I would like to thank the Centre controller on 120.15 and the CS Appr controller who was on late this afternoon for your help. Good one guys!
A couple of vectors and a descent to radar lowest safe got me safely down through the overcast.
B*gger it! Now I have to grope my way home tomorrow VFR.
Moral of the story is: if your vac pump needs higher than normal rpm to extinguish the low suction light - it may be about to sh*t itself.
Always carry a couple of covers for the AH and Gyro Compass. I was wandering around a bit until I wacked sticky covers (which I carry) over these.
Dr 
PS: Anyone have any experience with the new "super dooper" long life vac pumps (see Global Aviation) over the standard garden variety?