Brake fluid loss!

I'm suffering really heavy fluid loss to the point where i can pump the pedal for about 30 seconds and its gone down about 4mm on the reservoir! I can't see any leaks in the lines and there is no fluid patch on the ground, where can it all be going?!?!?!   

I'd guess rear cylinders but that would eventually drip on to floor. Have the brakes been fannyed about with (notice you have a 16v fitted).

Its possible for the master cylinder seal to go and the fuild to leak back into the servo and then into the engine, which mean there is never any trace of were its gone :dontknow:

Thas is what i suspect it is and fortunately, i've got 5 litres of brake fluid and a 16v servo/mc in my car which i picked up last week!