Cleaning Gunk Out Of Overflow Tank

I am flushing out the engine coolant in my 1995 Ford Mustang GT. I think I will use conventional Zerex since it is green and is suppose to last for 5 years / 100,000 miles.

Anyway, I looked at my overflow tank and noticed a bunch of gunk in it. I removed the tank and have been using HOT water and Dawn dish soap trying to get rid of that gunk. Most of it is gone, but there is still some in there.

Is there some sort of chemical which is safe to use in it to clean it up perfectly? I don't want to use a chemical that leaves any kind of residue behind.

Thanks In Advance,
Norvin

So far I have tried bleach (which got some of it out) and fish pebbles (to bang against the gunk to loosen it) along with HOT water and Dawn and still it is there. I wonder if muriatic marine acid / cleaner would work safely without leaving a residue?

Norvin

Well the Muriatic Acid did the job. The overflow tank is nice and clean. BUT... it ruined my black plastic overflow tank cap. Mine has a long tube coming out of it with a sensor on it. Oh well. Learn from my mistake; take the cap off.

Norvin