Sedra 5000 not pulling air?

[2frosty4u]

I have a Euroreef CS8-4 recirculating skimmer that uses 2 sedra 5000 needlewheel pumps. I recently had to replace 1 damaged impeller and ever since have not been able to make that pump consistently suck air. I cant for the life of me figure out why it will only pull air for short periods of time before the intake tube fills with water. I'm hoping that someone has some advice as to what is causing this and what I can do to resolve this issue. Thanx all.