I've always used kosher salt and denatured alcohol. It's works pretty good. You may have to administer more than one treatment up to several depending on how much the pipe was maintained.
I plug the shank, fill the chamber halfway with salt, then use a dropper and wet down the salt but not completly saturate the salt. Fill to the top with salt and wet the rest. This gives the entire chamber an even distribution instead of soaking the top and leaving the bottom dry.
In a day or two when the salt has completly dried out, remove and repeat. When the salt stops turning brown after drying, it should be as clean and fresh as a summers eve.