My FW system wouldn't pressurize and air in it. I finally traced it to the "strainer" on the pump. Similar to this:
http://store.hamiltonmarine.com/browse.cfm/strainer-in-line-1-2-120412-/4,24600.html
http://www.johnson-pump.com/jpmarine/products/watersystem/accessories.html

I replaced it and the problem has gone away, it may have been a small crack or just a poor seal. I'm not on the boat right now so can't reference a specific manufacturer.