brake probs- servo???

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i have a 1.4xe16v corsa b sport. it has calibre turbo callipers and discs and had real strong stop on a sixpence braking. when after it was stood up over winter i have no brake pedal goes soft to floor. thurlbys reckoned master cylonder. so they put on a new one and bled them up. still the same crap brakes! they reckon servo was fine but when i drove it today before mot when you drive there no brakes but when you left foot brake while still accelerating brake goes hard like norm. as it was fine before winter then all pipework is correct which leaves me to think duff servo!
any clues/ideas anyone? also whats the biggest servo i can fit straight on no hassles to my car as a upgrade replacement!!