skimmer main drain setting

When I lower water level with skimmer setting for main (bottom) drain only and rotary valve set at waste, water is still being drawn through skimmer as it is when set to just filter. Suction is less than at normal filter setting, but there should be  NO water going to waste other than from main drain, right?

How can I stop this and only draw through bottom drain when lowering water level below skimmer to close pool?