I am recording this to save others' pain.
Seems the brake master cylinder is on its way out in the MK3III TDI.
And that there are two sorts fitted to at least the TDI, with either 2 or 4 inlets holes, and different bore sizes, 20 or 22mm.
You are in luck if yours is the four port brake master cylinder as they are more common and around 40% of the cost of the two port ones.
Tooleys had both, most places have only the cheaper 4 port one.
Happy motoring