Brake problem...

You most likely have air in the master cylinder, this can cuse the piston not to work. I had this once on a trials bike, and one way to fix is to take off the whole system off the bike(keeping it sealed...banjo bolts tight), and leave it inverted, so master is low down, slave is high up. This will allow any air bubbles in the system to collect at the top, away from the master piston. Then whilst the slave is still high, put some fluid in the master resevoir, and pump until it flows out the slave bleed nipple. This is a basic bleed, and can then go back on th bike for a proper bleed.

another way is to 'reverse' bleed it by using a pump type oil can (Like from the Wizard of Oz!) attach this to the bleed nipple using a suitable pipe, fill the can with brakefluid, remove the master resevoir cap and pump fluid from bottom to top until all air is purged....you will see the bubbles emerge from the small hole in the bottom of the master reservoir(common trials bike trick)