That would be an incorrect assumption. Fuel isn't going to drain uphill, through your needle and seat to leave the carb. Yet when you go to start your car after it's been sitting for weeks there's no fuel in the bowls, requiring much cranking to refill them enough to fire the car. At starter rpm, mechanical pumps don't seem to refill the carbs all that quick. Fuel's evaporating out of your carb, it happens to all of us with this modern pump swill they call gasoline. A check valve won't do anything for you. My last two carbed vehicles I changed over to electric fuel pumps. All my fueling trouble vanished. Turn the key to run for a few seconds so the electric pump can fill the bowls, pump the gas and hit the key. Doesn't get any easier than that.