Rob, I have seen the same problem with the reproduction clutch master cylinders as well. The last time was about a month ago. Having seen it before I knew the problem was a poor seal at the foot of reservoir and there is no fix short of JB Weld. This time I looked in my box of old master cylinders and unscrewed a reservoir from a dead unit and swapped it onto the new leaky unit. It works perfectly and looks correct. Below is the new master with an old reservoir.
