I have a 99 F150 with a 4.6l v8, and an auto transmission. Yesterday I was doing my routine oil change and fluid checks, and I found that my coolant had a slight yellow film on top of it. Its the type of film that everyone describes when there is stop leak in the system. So I drained the coolant and flushed it once, and when I drained the system after the flush I noticed a slight sediment build up in the reservoir (in the picture below). Does this confirm the stop leak theory or should I start worrying that its head gaskets? All of the other fluids were normal, no white smoke, no coolant loss, system holds pressure, and I have never had a over heating problem.

 

Some sediment is not abnormal. I'd do what you did and keep an eye on it. I wouldn't sound alarm bells yet.

 

I had the same problem when I changed my water pump two years ago. I just took the bottle out and cleaned it. I changed all my coolent last fall and it was fine. I wouldn't worry until you see oil in coolent or very thin oil when doing you next oil change.