remove the pet cock. take an 1/8th inch welding rod (after stripping it)and rod it in there. it may take a while but you'll get it in all the way. swish it around while working it in. have plenty of water and patiences until there is nothing left to remove. then put some radiator flush in and follow directions on the label. every so often drain the block until no dirt is visible. thats what i had to do to three trucks. sounds like they didnt do a shake and bake before the rebuild