could be brake servo at fault- to check,with engine off,pump brake pedal several times to remove vacuum from servo,then with pedal depressed,start engine and you should feel brake pedal travel further down-if it dosent then fault is either in vacuum pipe from manifold [could be blocked where it connects to manifold],or servo itself-there is a seal fitted between master cyl and servo that can leak vacuum if worn.not sure if seal is available on its own !
k
April 2008