Jansin, Welcome to TRT!
If the air is actually bleeding from inside the pump, you should take the pump back. That shouldn't happen.
Before you do that, let's make sure the pump is the problem. The Iwaki will suck pretty hard on its intake, so any tiny little holes in your fittings will allow air to bleed in... even holes that are too small to detect with a water leak test. Suggestion: remove the water from the pipes, seal the ends, pressurize it using an air pump, then apply a soapy solution to all your joints. If there's a leak you should see bubbles.
I'm assuming, by the way, that you have eliminated the possibility of bubbles escaping from the skimmer.