How old is the M/C? Does the fluid in the other reservoir go down? I had a problem once where one reservoir of my M/C would fill up and overflow like yours, but over time the other side lost fluid. There were no external leaks. I finally figured out that the internal seals in the M/C were worn and fluid was being pumped from one circuit to the other. A new M/C fixed the problem. I think you may have disturbed the old seals in the M/C when you bled the brakes.
It is also possible that the seals in the combo valve are bad, allowing a similar transfer of fluid, but my money is on the M/C. Those are the only two places in the brake system where fluid from one circuit can flow into the other circuit if there's a bad seal.