What's wrong with my 4+ years old skimmer?

[h2o]

While rebuilt my tank bottom with plenum, I disconnected the hose of my Top Fanton 100 skimmer, rinsed it and reconnected it back and to my Rio 2100. In preventing leak I put extra telphon tape on the elbow pvc to the skimmer and the hose, but after turning it on it WAY OVER Skims and water just got push in to the waste cup and filled it up in no time. Is it the telphon tape, what could be wrong. I have these skimmer and pump for 4 years, never over skims like this, the air hose isn't plugged. HELP !

 

[mr . salty]

I had a similar problem with my CPR skimmer,,,It turned out to be a restriction in the return line.It had a dip in it.That was causing back pressure thus not allowing free flow back into the sump.This caused it to back up into the cup.The water was flowing in faster than it could flow out...