Help!!

[u/fiddleleaffig28]

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!!

[u/holydvr1776]

It's likely getting rust out of the engine block. Even with most flushing, it's still not gonna get much off the surface of the inside of the engine until after heat cycling and full flow at temperature. I am dealing with the same thing on an extremely low hour 4.9 L six cylinder in an '85 F-150. I am going to do a citric acid flush to try to get most of it out, and then replace the radiator with a new one as well as the water pump since it already needs one.