well, after you've checked all you have, the things that i would check is all fittings in proximity of the pump (can you hear air anywhere after you run the pump and turn it off if you stay by the pump?) can you zip around to your returns and listen for air leaks right after turning off the pump?

but now that i think about it more, given the air build up in your filter..that might be the next most obvious place you look..sounds like somewhere the pressure went too high..if normal operating pressure for my system was say 20, my filter was rated for 50 and i closed my returns and ran, i bet i'd get up to 50 plus really quick. could you have cracked a valve on top of your filter? when i had some leaks, i bought a bottle of this spray on leak checker at the hardware store. when you spray it on anything that has any airleak, it immediately will show bubbles/foam where the leak is to help pinpoint. a small bottle sprays quite a bit of area and it worked great for me.