When I bought my Phaeton, it had been stored for about 15 years. The clutch was rusted to the flywheel.
From the MAFCA Website Technical Tips... I got the following Fix for a stuck clutch -
"I put the rear of the car on jackstands and left the rear wheels free to rotate. I then started the car in 1st gear and ran it a minute or two. I accelerated the car to about 20 mph on the jackstands. Then I depressed the clutch and hit the brake fast and hard. It broke loose! I engaged the clutch a few more times on the stands. When I put the car on the ground, the more I drove it the better the clutch worked. I sold the car to a friend of mine who put several thousand miles on the car with no problems.
--
Chip Evans "
It worked wonderfully. A few thousand miles later, I have never had any trouble out of that clutch.