Organbuilder,
There is a soft plug (freeze plug) near your crack. If you are thinking about flushing some of the crud out so the crack will stop weeping, you could knock the plug out and flush from this hole to the petcock or vice versa! The crack area is right at the bottom of the cavity and is probably a swamp.
You drilled some stop holes in the crack so when the gunk is washed out, clear water should drain from these holes. You can buy a expanding type freeze plug to replace the hammer in type and it should be fine.
Stuart
