[Woody]

I have had this problem before as well. I have cleaned the top lip of the reservoir (it was slightly rusty) to remove the crap and it sealed. it is leaking again but this worked for a year or more.

Also you may have resurface the top lip of the reservoirs to get a better seal. To do this you need to empty most of the brake fluid from the reservoirs and then stuff them with a rag or paper towel. Use a flat file or sandpaper wrapped around a flat piece of wood and file/sand the top until smooth. Clean up the mess you made. Next make sure the MC lid to straight by laying it on a flat surface. Repair as needed. Remove the rudder gasket and ensure it is clean on both sides. Then refill MC with brake fluid and replace the lid.

That should fix your problem. Good Luck.