What you are describing is the typical failure mode of these servos. I would guess that you have a Girling MK IIb from what you are describing but I would have to see a photo to be sure. Often these have been replaced with a variety of othere remote type servos.
If it is a IIb it can be rebuilt though I think that it is much more difficult than the IIa because of the 'clamshell' nature of the vacuum pot.
You should also be aware that the kits currently available are often of poor quality, or if NOS so old as to be useless (if you can find one).
There are several things you can do, IMHO the two best are to replace the servo with a remote lockheed (expensive), or simply remove the existing servo and replumb(cheap).
When I get the chance I will take a photo of what I used to bypass the servo and post it to this thread ...all the parts are available at any decent hardware store and are inexpensive.

Good luck & Cheers!
OK