If the piston is tight, you will need to see to it. Hone the cylinder only enough to remove any pitting. Replace the seal or cup.

Overhaul or replacement of the m/c at the same time may or may not be necessary. If fluid is flushed regularly, it is highly likely that niether one will ever need any attention. Since the slave DOES need attention, then the m/c has operated with the same dirty fluid, so it might. It would certainly be a good time to check it.

Good luck,