between the 1930s and 1950s also used engine-driven exhausters. They were designed with vacuum braking systems or servo-assisted brakes, based on earlier models with petrol engines. As petrol engines produce a manifold vacuum, vacuum systems are easily added. Diesel engines have no throttle or manifold venturi, thus do not provide a usable vacuum source. Trucks and later buses used compressor-driven air brakes instead. Dual brakes Vehicles can be fitted with dual brakes, vacuum and air, provided that there is room to fit the duplicated equipment. In a dual fitted vehicle, there would be both a vacuum cylinder and one or more air brake cylinders, all operating on the same set of rigging to apply the brakes at the vehicle wheels. Some of BR's Mk1 coaches were built with dual brakes (all had vacuum as standard) and much of the rest of the fleet was dual fitted by the 1980s, so they could be worked by air or vacuum fitted locomotives as the changeover from vacuum to air took place between 1970 and the early 1990s. On a smaller vehicle such as a traditional four-wheeled goods wagon, it is much easier to fit just one kind of brake with a pipe for continuity of the other. Train crew need to take note that the wrong-fitted wagons do not contribute to the braking effort and make allowances on down grades to suit. Many of the earlier classes of diesel locomotive used on British Railways (and electric locos up to and including the Class 86) were fitted with dual systems to enable full usage of BR's rolling stock inherited from the private companies which had different systems depending on which company the stock originated from. Air brakes need a tap to seal the hose at the ends of the train. If these taps are incorrectly closed, a loss of brake force may occur, leading to a dangerous runaway. With vacuum brakes, the end of the hose can be plugged into a stopper which seals the hose by suction. It is much harder to block the hose pipe compared to air brakes. Twin pipe systems Vacuum brakes can be operated in a twin pipe mode to speed up applications and release. Twin pipe vacuum systems were standard on the first generation British Rail Diesel Multiple Units which replaced steam locomotive hauled passenger trains on many branch and secondary lines in the 1960s. The second