You can get all kind of junk on the lower back side of the number 6 cylinder and that will definitely keep it from draining. The last time I had this happen, I had to remove the water pump, pour the opening as full of CLR as I could, let it sit overnight and then bend an L shape into a coat hanger and work it from the front to the back to dislodge the stuff. Took a couple of hours. Finally I got the petcock opening to drip a bit and then I poked a coat hanger up there to help dislodge things. Then I drained that CLR, replaced it with new and repeated. After doing this three times, I got it fairly well cleaned out. I remember wondering if one of those spring type pipe cleaners would help but the one I had was too large. Some are smaller (for smaller pipes). That might be ok, but I haven't tried it. Jerry's idea sounds good. Good luck.
