Bubbles in the pump basket area will ALWAYS be a suction side issue (before the pump), not pressure side (after the pump), so it's unlikely a filter problem.

You can try isolating the skimmer, then the main drain, then the vaccum line to see which creates more bubbles than the other.
If it doesn't change, it's probably between the diverter valve and the pump.

Take a dripping wet dish towel and wrap it around glue joints, valves, and fittings, one at a time and see where the bubbles reduce or stop. That would help isolate the leak.

Hope this helps,.