Magnum 350 canister filling with air

[adeebm]

So I have this magnum 350 canister filter that used to be on another tank. I took it out, cleaned it, and set it up on a different tank that needed more filtration. The tank sits on the floor at the same level as the canister. I fill the canister body with water, burp out all the air, and start the thing. At first it runs fine, but over the course of the day, it fills with air, until eventually it can't function anymore. There's no water leaking out, but there must be some seal through with the air is going in. I checked the o-ring around the top of the canister and reseated it to no avail. It must be either the o-ring, the quick connect valve, or one of the seals on the intake tubing. Any idea how I can find the problem and fix it?

--Adeeb