It is probably the machine marks causing it to leak. There is also nothing holding the piston into the slave cyl except, when installed, the fork. The last cylinder I had was also from Hervey and it failed in the same way yours did (leaking out of the boot and ultimately all over my clutch!). I needed a replacement and wanted to buy from Rob but his cylinders were not ready at the time. I decided to buy the cheap 'Euro' slave cylinder from DeLoreanGo. It has been trouble free and it's actually even cheaper now than when I bought it in 2017.