I'm not sure how long you owned your car, but another possibility for the brown sludge is radiator stop leak. After circulating through the cooling system the brown sludge eventually ends up in the overflow tank.
Chevy puts radiator stop leak in their new cars - I guess as a preventive measure.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles