[MazdaX]

The hydrolic master/slave are designed to work as soon as you start pushing the pedal in. If you are slipping now it's because through full plate / disc contact was not happening before but it is now and it's causing break down of the disc.

That would be my guess. It's not really possoble to cause your clutch to slip from replacing those two items inless they did something stupid like forgot to extend the piston in the slave.