Originally Posted by Joe@ZSpeed

Yes, It must be replaced, At least with the stock unit. When the trans is installed the slave is compressed, when you remove the trans the slave extends all the way out collecting all the debris (abrasive clutch particles) into the seal of the slave.

We have had a few people reuse our HD version after pulling the trans without any issues although we recommend changing it if it has been in for a longer amount of time, We offer a new cylinder by itself (no lines, adapters ect) at a lower cost, or you can send the whole thing back and we can replace/rebuild it for you.

I just had one of our long time, one of the first users, of our HD slave kit contact us about our Wilwood master cylinder upgrade as his stock one is wearing out and this is what he had to say about the HD slave kit.

Quote..
" Not sure if you remembered me, was 1 of the 1st few to use your metal csc, i'm located in Singapore its a fantastic product, lasted me 2 rebuilts of my os giken twin plates total around 50k miles with heavy usuage in drifting"

Pretty sure he is still using it now.