No service station shall be permitted where any oil drainage pit or visible appliance for any purpose (other than gasoline filling pumps) is located within 20 feet of any street line or within 50 feet of any residential district. All such appliances or pits, other than gasoline filling pumps, shall be within a building. Gasoline pumps shall be permitted within the required front yard space of service stations, but shall be no closer than 15 feet to the street line.