Leak at brand new clutch master cylinder

I changed the slave cylinder and put in a new release bearing just to be sure. In the meanwhile I also changed the rubber gearchange bushings by new yellow ones and I put in fresh gearbox oil.

I also had a look at the brand new clutch master cylinder

and found out there was a small oil leak ???

 I dismantled the master cylinder and gave it a good clean and you can see some small traces in the cylinder.

I put it back together and it doesn’t leak anymore so I guess there was some debris from the old slave cylinder between rubber cap and cylinder wall and the small traces are not big enough yet to  make it leak.