1995 F450. Installed new radiator less than 2 years ago. With old radiator in place, drained cooling system, flushed, put in rust remover/flush, followed directions, rinsed system and put in fresh coolant mixed with distilled water. Noticed coolant in overflow tank changing color. Removed radiator cap to find brown blob that reminded me of silly putty. No overheating issues even with temps in the mid 90's now. What needs to be done to keep this from happening? TIA.