Abstract: An activated charcoal filter is placed in the vacuum conduit between a brake booster and the point where the conduit is connected to an engine to tap intake vacuum for booster power. When for any reason fuel vapor from the engine tends to flow toward the brake booster the charcoal of the filter adsorbs the fuel vapor. When flow of air returns to the normal flow direction, from the booster to the engine, the fuel vapors are desorbed and returned to the engine. Thus the filter has a self-purging action. This effectively prevents condensed fuel vapor from adversely affecting elements of the booster such as the diaphragm when under some particular circumstances and in certain installations fuel vapor would otherwise be introduced into the booster and condensed.