Mine has exactly the same symptoms you described Brian, but a difficult close inspection revealed the seal round the pushrods from the pedal has popped out of its housing. The daft thing is the brakes are fine as it's just been tested but the servo leaks. A mechanic has said it could get no worse for ages but then again may fail completely. For safety I'd like to sort it. There's plenty of second hand servos on flea bay but recent ones are from later models and/or diesels. I wondered if anybody else had changed one.