I would have expected the PSI to drop if you closed off the skimmer or main drain. It sounds as though maybe your new filter PSI guage is not working properly. This is a fairly common problem although not so much for new filters.

Just to confirm, you did not see PSI change with either main drain or skimmer but did you set the valve for both main drain and skimmer on? Any change there? Have you checked to see if there is any blockage in the lines. You can backflush the lines with a hose and a rag (to seal) or get yourself a Drainking.

Also, about how high is the pad above the pool? Although this does not add to the total head loss, elevation will increase the suction head and decrease the return head relative to no elevation.

1.5" plumbing is really too small for the suction side and could be primary cause to the air in basket and low PSI reading. Even though you new pump is only 3/4 HP, it is still a very strong pump and possibly too strong for your plumbing. The pump cannot draw enough water so it is drawing air through the seals in the pump lid or valves. A larger pump would only make this worse so don't go that route. However, a two speed pump would allow you to run at the low speed most of the time which your plumbing could easily handle.

If you want to go to the trouble of measuring the suction on the pump, you just need a vacuum guage, Sears has one for $20, and get an adapter so that it can screw into the pump's drain plug at the bottom of the basket. But this will only confirm that your suction head is very high and not really tell you why.