What make and model car? What's OEM coolant? "Brown stuff" could also be Dexcool when it is exposed to air.
Bar's Leak is a good stuff. Subaru requires adding it to their cars, during coolant changeover. Subaru boxer engines are prone to headgasket leaks. It is a shredded tree root.
Do you have any coolant in the oil? Look for milky residue. You can also pick up an oil analysis kit from Amazon for under $20 and send oil for analysis. They will be able to tell if there are traces coolant in the oil. One of the labs in in Atlanta, so you should get results fast.
Leaking transmission radiator is another possibility. Check tranny for high fluid and signs of coolant, same as oil.
Replacing coolant with fresh staff will not "flush out" the plug, if Bar's Leak was added. If in doubt, add some more, but go by the bottle recommendations, do not dump the whole thing in. Do not use All Makes/All models coolant. It is a Dexcool clone and should not be used in non-Dexcool cars. 2EHA acid it uses is nothing to play with. BTW, draining radiator will replace only 50% of the coolant in the car, so you will need to do it 3 times to get most of the old stuff out. I normally drain the radiator, fill up with distilled water, run the engine untill it is hot, with heat on to work the heater core and do it free times. drain, fill up with straight coolant (50% of the system capacity), run to mix, same as above and test the concentration with hydrometer and coolant level overall. If you are at 50/50 mix, but low level, mix up a 60/40 top off bottle and use it to fill to level.
The oil looks like it could use a change soon but, it's not milky. What is Dexcool? I might try this Bar's Leak stuff in my car too might help my low compression. lol might not. Any way, thanks man. I'll do it now knowing it won't flush the plug out if I change the coolant.