Heinze, when you get the head off many things will become apparent. Once the head is clean a close inspection with a magnifying glass may do the trick. If you find nothing in the block especially at the bottom of the cylinders where they sometimes rust through, look for a pin hole.
If the block is OK and you "SEE" nothing cracked on the head have it magnafluxed at an engine shop. Cracks often form around the alternator bracket!!!!
DO pressure check the manifold again!
Dave Neptune