Just some experiment I been doing by accident.
Due to some plumbing problem, yesterday I used my back up on trolley Pentair 1.5HP Superflo + Pentair cartridge as vacuum for my pool.

It has reinforced hose ( 1.5" ) of 33 feet and I connect 2 sets of vacuum hoses, I think the vacuum hose are like 30 ft each, I am guessing.

So with total of nearly 100 feet, I am not able to get suction. So much bubbles in the pump strainer.
I removed 1 vacuum hose set and down to about 60 feet. I can get suction but with consistent bubbles. All hoses connection are underwater and vacuum hose is completely submerged, so no air leak. The bubbles are a lot. Filter pressure is at 5psi and since I have a flexible discharge hose, I squeezed it to create back pressure on the filter, when the gauge reads 10 psi or more, the bubbles slowly dissapear.

So I guess even without a leak, air bubbles can form when suction ( Hg ) is high or when water flow is high. Those bubbles I see, am i looking at cavitation bubble or what ?

Without the extension of any vacuum hose on my suction side, no bubbles.

I think here the importance of having a properly sized suction pipe/hose and as short as possible is very important as the pump guide suggest.

On my fixed mounted Hayward 2.0HP Super Pump, I am getting the same problem. If I only use the 1.5" main drain suction which is about 50-55 feet in total length, my vacuum reading is incredibly high and I will get air bubbles no matter what. I thought I had a suction leak but when I compare to my back up Pentair on trolley, I think its just too heavy suction for the Hayward. If I open both the balancing tank 1.5" valves and the Main Drain, then the bubble will go away.

What do you think guys ?