This morning, before starting the final leg on a trip to Southern California, when I checked the water, there was a layer of dark brown material on the radiator cap seal. When I got home 180 miles later, the stuff had been deposited again. It hadn't been there before, but I seldom go that many miles between checking fluids. I just reinstalled the engine about a week ago after being out for about three weeks, but didn't do anything to the motor's cooling system while it was out. I'm using the same brand anti-freeze, Prestone, as before. The aluminum radiator has been in for a while. The only thing new to the cooling system is transmission cooling lines to take care of the C 4 in place of the FOM.

Anybody have any thoughts on what these deposits might be? I doubt it's the trans cooler as there would have been water in it when I uncapped it to install cooler lines.

Thanks,
Don