I know you can tell I'm a newbie by this question - how do main drains work?
I know (because I can see them) that we have two main drains in the bottom of our pool. But they don't seem to 'do' anything. Shouldn't the pump be pulling water from them? I see small bits of debris on the bottom of the pool, but they don't seem to be moving towards the drains, which seems to indicate to me that there is no movement through them back to the filter. Is there some setting - on the filter valve, maybe - that I need to set to get suction from them? I am getting plenty of suction through my new skimmers and I can feel powerful jets of water returning to the pool through the new 'eyeballs' I put on the returns. But I can't tell if anything is, or should be happening down there on the bottom.

35,000 gal - 3 ft to 8.5 ft
Sylvan pool (mid 70's maybe)
shotcrete with new plaster/marble finish
1.5 hp Hayward Super pump
Nautilus NS 36 sq ft DE filter
Polaris 380 with 1hp PB4 booster pump
two skimmers/returns
two main drains
one Polaris port