RODC 5500S skimmer pump questions

[Rezac]

I bought an RODC-5500S pump for my diablo 300, It pumps water like crazy but it won't make any bubbles, I attached an air intake line to it and it still doesn't. The only way I can get it to is by slightly covering the water intake, it isn't a lot of bubbles but it is some. Coralvue said that the only replacement pump I can use is the HY-5000. They both have 1-1/4" outputs. My RODC doesn't seat as tight as the PSKs do. I could try an o-ring inside there since there is no room for one otherwise I suppose. Anyone have any ideas?