I think it's the master and I'd have a word with Karl to see if he can let you gave a s/h mc at a reasonable price.
On the same theme, moons ago I had fluid leaking into the vacuum diaphragm chamber of the servo. It was a petrol car, so the servo was vacated by a hose to the inlet manifold. I must have been topping up the master with fresh fluid quite regularly (it wasn't an uncommon thing to do in the old days) to he extent that the servo had filled right up to the vacuum hose, with fluid.
I only found out when stopping sharp at lights or a zebra, and I'd be overtaken by a cloud of white smoke big enough to block out the whole street!
I'm not suggesting you have the same problem, but there were no external leaks, and there was creep.