rust on top of master cylinder

Just replaced left rear brake cylinder and now 3 weeks later, fluid is low in front reservoir. I noticed wetness down the front edge of master cylinder basically from top downward. When working on it for left rear, we noticed some rust on that top edge. Would this (improper seal?)cause a dribble down over? Never did that before. Is there any pressure in there during use?

Wow, I just had that problem with my jeep, and I also changed the left rear brake cylinder! I took it to my grandfather's brake specialist and he finally got the answer after 4-5 visits.
Well, there was many little problems that got together to cause the problem:
1:The left rear cylinder was broken so, brake fluid scaped trough there. That appeared to be the brake fluid leak.
2:It continued to loose fluid (even the front reservoir was almost dry), but only the front reservoir. The master cylinder cover was slighty bent and was loosing fluid from the front(but it hadn't rust on it). Problem solved. No.
3:The problem continued. We replaced the seals and O-rings and the ghost leak dissappeared for a while. But at the end it reurned!
4:The solution was a small O-ring that was old enough to let the brake fluid pass from the front to the back reservoir. So when opened, the fluid seems to overfill the back part and leave a few mm of fluid at the front.
Since the it had not been loosing fluid and was, apparently the solution for the mistery. By the way, this was not ABS equiped unit. Hope this helps a bit. Good Luck!