Had this a fair amount on old 911's, it can be quite common when they get hot....First step is to bleed the system, get rid of any trapped air, next step is usually rubber seals in the cylinders.
I replaced a slave cylinder on a Vr6, which was cheaper than the last step which would have been the master cylinder.
Someone with more understanding of a mk 5 than me will know if the car has a slave cylinder (i would expect so).