I'm doing a flush of the cooling system and when undoing the engine drains on the sides of the block one was stuck hard enough that I left it and the other opened but was full of crud. The drain that opened was the one on the driver's side that was like a cap on a threaded tube (the drain on the other side just looked like a bolt.). The tube was packed full of dry black stuff like dirt (smelled like antifreeze). Is this rust that the antifreeze has 'treated'? I managed to pick some of it out of there but either I've hit the end of the tube dealy or there's a lot more crud in it. Anyway, has anyone else encounted this and should I keep mining away on the drain until some coolant flows out? I'm going to back flush the system and then run a prestone cleaner through it before refillling it. Oh yeah, it's a small block 400 if that makes any difference... Thanks in advance!