I warm up (50degC or so) some clear white viniger and fill mine about 2/3rds full. Give a good shake and leave to stand for about 10 minutes. Shake some more and leave to stand the other way up.
Empty and rinse with de-ionised water thoroughly empty and fill 2 or three times.
If the viniger was not completly clear coming out, do it again

. You'll see all crap that would otherwise have ended up in your loop once it warms up.
I know some people go ARGH VINIGER but honestly, it's fine. My loop hasn't had an issue at all doing this, and it will strip off any axidisation on your copper rads. Just make sure you thoroughly rinse it out afterwards and you're golden.
I'd do it maybe a day or two before you want to fill the loop personally. Drying isn't ultra essential so long as you make sure you rinse them properly.
Disclaimer: If you put hot viniger in a radiator, it's gonna get hot, because it's a radiator and that's what it should do, don't forget this like I did the first time and go "OH SHIT that's hot!" lol