Thanks chaps...
It was the reference to water that threw me. The part in question is 31 years old, so I guess it will need monitoring. No sign of brake fluid on the floor yet. So far the brakes feel fine. Had both front calipers bind up the other day, but sprucing up their pistons sorted that.
As they said "It's nothing to worry about".
Getting old is no fun!
The one advantage I've noticed of taking a car as old as mine to a dealership is that they no longer carry 99% of the parts needed, so they are unwilling to repair faults they find.
