If that doesn't work check the piston. If the piston and both rubber cups are ok then check the supply and relief ports in the master cylinder and make sure they're not blocked. more so the relief port than the supply port.....Angelis

1200CC BIG BORE, W/WISECO PISTONS,.250 STROKED CRANK, PORTED/POLISHED HEADS AND LARGER VALVES, CUSTOM WELDED EXHUAST, DUAL 42MM MIKUNI CARBS.