A plastic pulsation bag for automobile gasoline filters and a method of making the same is disclosed. The bag comprises two opposing flexible membranes peripherally sealed together along their opposing sides. At least one of the opposing sides is recessed so that a pocket is formed by the sealed membranes for enclosing a gas which is preferably heavier than air. In a preferred method, a first step in forming the bag is to slopingly insert the membranes into an open-top chamber containing a body of gas, such that the recessed opposing sides pass obliquely through the upper surface of the gas and face away from the chamber. The membranes are then agitated within the chamber to release trapped air adjacent thereto, oriented in an opposing relationship, mounted upon opposing sealing dies, and then sealed together about their peripheries. Pulsation bags made in accordance with this invention are useful in preventing "water hammer" in certain large vehicle fuel systems.
