Why does the air stop blowing out of vents when you step on the gas to increase speed?

If you included the kind of car you have,I could be more specific but all cars have a vacuum canister under the hood. some are round plastic and some are a tin can and most are black. There shuold be a vacuum line going from the intake to one end and a vacuum line out of the other that supplies vacuum to the interior of the car, There should be a one way valve in the canister or the line from the intake. This valve is not functioning and loosing vacuum in the resirvoir. If it is in the resirvoir which most are, replace the resirvoir.