I would say it is an urgent job before you loose clutch pressure altogether.

I have found a link ( attribution to the UK VW T4/T5 forum) that provides good details per Master and slave cylinder and potentially all the replacement parts. I am not sure these days whether the cylinders are rebuilt with new seal kits and some honing of the bore or it is straight replace the entire part. I haven't had a manual for a while!

Suggest you obtain a separate quote from your service guy to your major service quote so you know what you are up for.

Clutch Master Cylinder Change | VW T4 Forum - VW T5 Forum
Clutch Master Cylinder Change | VW T4 Forum - VW T5 Forum

Regards,

Scott