Claude,
If your brakes haven't really been used for 18 years, and you have been using mineral brake fluid, they may need rebuilding again.
Mineral brake fluid is hygroscopic - it absorbs moisture from the atmosphere, so it's possible that the skirts of the pistons have corroded, like the attached photo (this is what the pistons from the front calipers in my 411looked like).
There's really only one way to know for sure, take them apart and have a look.
This is what they look like
Resources - Bristol Cars - Owners and Enthusiasts Forum
Kevin