Whole-house water filter leak
 

My house has 3 of these in series:

http://www.waterfilters.net/assets/images/hf-360.jpg

The last one is leaky. I tried using a spare housing, I replaced the O-ring, and greased it up with faucet grease. Is there anything else I can do?

When I bought the house it was fine. After the first or second filter change it started leaking a little bit -- I put a pail under it and never had to empty the pail. After this last change it's probably leaking a gallon per day. :(