Why So Many Air Bubbles at Start of Pump

My inground pool gets a lot of air bubbles via return inlets everday when the pump first kicks in. The bubbles continues to blow in for almost 50 seconds before it settles down. The system otherwise works fine.I have checked all above-ground equipment (no heater) and plumbing for possible leaks without luck (not sure how to check the DE filter container).

Why that is? Any ideas/suggestions?

Still puzzled! See Video - https://www.dropbox.com/s/mvxq23srxm3h68a/Pool%20Bubbles.MOV?dl=0

My guess…

The system has a suction leak. When pump is stopped overnight, the leak emptied the water and created cavity in the pipe line. When pump is turned on the next day, the pressured water takes the air with it and thus the bubbles from pool inlets. Make sense?

A leaky filter tank - Filter tank is almost empty of water when pump 1st kicks in. It takes a long while for the pressure in tank to build up. The bubbles may be a result of air being emptied from filter tank. Possible?

Your thoughts please.

I just built a pressure test gear to see if there is a leak in the suction line.

https://www.dropbox.com/s/v42xpyb1yw7ipip/DSC06240.JPG?dl=0

The gear should work with my garden hose! But my challeage is at the skimmer - what to use to block the Main Drain to complete my test?

https://www.dropbox.com/s/sxv0z5rxghwxmm4/skimmer-anthony-old-style-plumbing-5.gif?dl=0

Please help!