First , possibly it would be best if you posted a photograph of the Master cylinder , with the S3 Lightweight - depending on age there are two types of Girling MC depending on vehicle age. There are pattern MC around - I would not use one.
You say you are not getting a loss of fluid - from front & rear compartments of the MC ?
It is possibly to get fluid loss from the MC , a bad case you would get it running down the pedal box - look for dampness.
The Girling Supervac (genuine) at the mounting flange , there is a small drain groove , feel underneath with tissue - check for fluid loss under bolting flanges. Where the MS sockets into the booster can get corroded , so rust can choke up this drain-way - the only check is to remove and examine.
You can get seal recon kits , depends if the MC is worth salvage.
Basically there are early & late vacuum check , manual or the later auto on ignition start , often these don't work.
Doing the standard pedal check , vacumm exhaust - pedal should drop abt. 1" and then be rock solid.
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Look at the pressing between , pocket for the MC mounting spigot - you will see a very small vee groove impinge (above vacuum switch) - this is the fluid drain if seals are worn.