Abstract: A hydrocarbon vapor recovery storage system including a charcoal canister for storing hydrocarbon vapor and a control valve for establishing the rate of flow of the vapor into the intake manifold of an engine. The system further includes a regulator (30) comprising an inlet passage (52,54) for receiving the vapor from the canister and an outlet passage for communicating the vapor to the control valve (22). The regulator further includes a device for regulating the maximum flow rate of the vapor comprising a piston (120) movable relative to a control orifice (64) to regulate the flow therethrough. The piston (120) is movable by a material, such as a silicon rubber compound that is capable of changing its physical size in accordance with the concentration of hydrocarbons in the vapor communicated thereto.