if it was root beer then you should change out all plastic/rubber components it the keg and serving line. RB tends to latch onto those things and stick around for awhile.
my usual mix is a scoop of oxyclean 'free'(no perfumes) and fill with hot tap water, pull all fittings and drop them in the mix. soak for a couple hrs then scrub with a brush, rinse and reassemble.
I use hot water and oxyclean free as well, I replace my post and tube o-rings every cleaning because I have a ton of them sitting around, I scrub the inside of my kegs if its needed with one of those nylon loofa things I bought just for cleaning the kegs. it might be a good idea to replace the line and lid o-ring as well as stated above, stash em in a bag for later soda duty.