Thanks for all the advice. I am pleased to announce that I have a working head! Instead of just pushing in, my vacuum breaker actually has a nut that tightens down on the rubber grommet, causing it to expand so it wasn't leaking there. The leak (once I was able to get a mirror up behind the back of the head) was actually coming from the water hose that was old and cracked/worn out at the end where it attached to the vacuum breaker. I repaired it and...tadaaa.....no more leak!
Now, I just have to fix the next leak. This one is in the shower line....
Thanks again for the advice.