Wot caused leaked brake fluid?

my car was motd end of sept 2010. however i decided to change front brake pads as they just ped. i compressed the piston on 1 calliper and fluid seeped from rubber seal. the brake lines are not leaking and when i removed calliper there was no sign of leakage as i had pumped braked only 5 mins before i am not losing fluid either. however i did not remove cap from master cylinder which is a sealed cap and there is no leakage there either. could not removing cap have caused compression and the seeping?????? this has got me stumped!!!!