while troubleshooting this i noticed that i get a little bit of air in the pump strainer basket when running MD only.
i have 1.5" plumbing all around, skimmer & MD on intake and 3 returns. when i run both MD and skimmer, i have absolutely no air in the basket and it all seem transparent - no visible turbulence. the same on skimmer only.
when i change to MD only i hear a "ooosh" sound and immediately i can see air in the basket. it seems like it gets drawn into a basket very quick, almost like it comes from under the lid. which it might when the pressure drops when skimmer valve closes. the amount of air in the basket goes down, but some of it stays and i could see some turbulence. i don't see the column of water coming in and being thrown around, but i could see how the water moves.
MD+skimmer: 11.3 psi
skimmer only: 10 psi
MD only: 9 psi
now i wonder if i do have an air leak in the MD line or maybe my pump (SFHP 1.1) is not powerful enough to draw enough water from MD fast enough? i hope it's the latter