Hi Ya Neil
im very sure the cylinder should be fine i would make sure there is no air in the system with a good bleed nice fresh brake fluid being added should help lots , then if that still does not help cure your problem check for any bad hose`s that may have gone internally if after those two checks you still have not a good pedal then replace the master cylinder .
naturally if any parts you feel are of an age or chance of wear then play it safe and fit new in them areas where required brakes is a must ! Have in times of emergency so never worth the risk
hope you have luck and keep us posted
kind regards
robert