79 s3 leaking brake fluid

I've got brake fluid leaking in the foot well just behind the pedal. got no manual so cant see was seal will have gone.

tandem braking system - servo assisted....wot els d'yer need 2 kno

If you're lucky it's the join between the reservoir and the cylinder - I think it's connected with a rubber seal. If not, the master cylinder is probably leaking. If the master cylinder is anywhere near original then replace it - spend the cash on a decent original part (Girling?). You can get overhaul kits if the cylinder is in decent nick but its a fiddly job and needs doing in clean conditions. If your cylinder is old it's probably not worth the hassle.

It could also be the clutch

Good point....

Mine did exactly the same few weeks ago, it was the seals on the master cylinder, I just changed it, that was easier than messing about, did the rubber pipe whilst I was at it, easy enough to do just a bit of a fiddle getting it in, one of those jobs you need fingers 7" long ! Bleeding took a bit of patience as well, all told 2 hours of a job maximum.