For vehicles with automatic transmissions, inappropriate material is used for the check valve that is to maintain the negative pressure of the vacuum brake booster. Therefore, there is a risk that the valve may stick closed while the vehicle is parked with the engine stopped and that the valve may fail to open, when the brake pedal is depressed while the vehicle is driven at low speed right after starting with the accelerator pedal not depressed. If these occur, the negative pressure of the vacuum brake booster may become lower than normal, when the brake pedal is depressed for the first time after the vehicle is started, in which case, the braking efficiency might be reduced.