I just built a pool at my house and when the vac is running I see air bubbles coming from the suction line into the pump. When I switch the valve over to the skimmer it stops so the problem is with the suction pipe coming from the vacuum connection (it's a dedicated line 40' long with 2 sweep 90's and then connect to a Jandy valve prior to entering the pump ) After doing the basic troubleshooting;
Checking hose for leaks
checking pump and valve for leaks no improvement.
Tried different pump speeds, lower speed helps a little but doesn't stop it
I suspect a possible restriction in the pipe so I removed the Hayward thread in wall fitting from the pool to look inside pipe and bubbles stopped!! . Reinstall the fitting and the bubbles return immediately
It appears that any restriction causes bubbles