vauxhall astra 2001 hydrualic clutch?

Had new clutch fitted along with master cylinder,took garage 1 1/2 days ,the clutch when pressed to the floor and released quickley does not follow your foot back at the same speed it seems to suck air which i think is a seal problem(the brake/clutch fluid resovoir cap is on tight),been back to the garage twice they say its worn pedal assembly? car only done 77500 miles ?the clutch when it went was sudden there was no slip or anything,i feel again it was a seal which broke down,went to main dealer they want £40 to book a appointment to look.
at it any help ? paul