I'd go for the master cylinder as well.

My vec did the same as I came upto a roundabout. Pumped the pedal & it worked ok..no signs of leaking (as it wasn't..it was just pushing fluid past the seal). Luckily I wasn't booting it.

Get it checked properly mate. The cylinder may *just be on its way out & therefore it is mostly ok and working fine (like the rest of your journey). Have you noticed any lack of performance on them since you've been driving it, or have you literally JUST bought the car? My vec brakes seemed to be slowly getting worse, but thought I needed some new discs/pads: once the cylinder got replaced, brakes were excellent again. No brakes is the last thing you want..especially if you really need them in an ememrgency