r/F100

I am here to ask a probably very basic mechanic question, but this is the first vehicle I’ve ever worked on, so give me some grace. I recently replaced the radiator in my 1970, and spent a decent amount of time flushing everything out. Yesterday I noticed coolant leaking into the cab of my truck (I went and purchased a new heater core today). I went to drain the coolant so I could replace said heater core, and my three day old coolant is brown. I don’t know if I wasn’t flushing coolant properly or if there is a bigger issue at hand. Any advice is appreciated!!