New Problem, Leaky MC Lid, Need Help

[Stocker]

Have had a rash of problems lately with the K20. Yesterday the front MC reservoir started leaking fluid around the lid and I can't figure out why. It's not overfilled, and it's never leaked before. MC is just a few years old. I wire brushed the inside of the lid and replaced the (rubber?) seal but no help. That's the reservoir for the rear brakes. The shoes and wheel cylinders were new a year or two ago.

I'm a bit frustrated here, been dealing with other issues too, and I have no clue why brake fluid is leaking around the lid. Any ideas? Thanks, guys.