[JFP in PA]

I really is not important if the valve faces pressure or vacuum, only that it works when power is applied, and that when it is open, there is flow through it.

The EVAP system is supposed to absorb (actually adsorb, but that is another science lesson) hydrocarbon vapor accumulating in the fuel tank onto the activated charcoal in the cannister; under the correct operating conditions (warm engine, high intake system vacuum), the purge line opens, sweeping air thru the cannister media and pulling the hydrocarbons into the intake to be burned rather than released to the atmosphere as they were with the vents on older fuel systems. The the lines need to be clear, and the valves have to open on command from the DME to get rid of the vapors.