Parking maintaining apparatus for brake booster

A parking maintaining apparatus includes a solenoid-operated open/close valve which opens or closes a constant pressure passage providing a communication between a valve mechanism and a constant pressure chamber of a brake booster. A relief valve is provided which relieves a pressure within the constant pressure passage into the constant pressure chamber when such pressure exceeds a given value. The provision of the relief valve allows the pressure within a variable pressure chamber to be maintained at or below the given value through the valve mechanism under a parking maintaining condition, whereby the pressure within the variable pressure chamber can be rapidly displaced as the parking maintaining condition is to be terminated, thus eliminating a partly braked condition.