Previously, heavy piston cylinders were used to implement pressures of max. 8-10 bar for the spreading of the brake shoes. At the latest, with the appearance of disc-brakes, can be seen, with how much less weight and installation mass the diaphragm cylinder achieves the same effect.
The chemistry has developed much further. The reliability of the diaphragms is so highly estimated, that one gives it the assignment of many thousands of braking processes. What's more, also even on the front axle. The pressure flows in from below (see the above figure) and pushes the diaphragm, together with the piston-rod, to the left. If the pressure is released, the (red marked) spring can push them to the right again. At the same time, the diaphragm provides the sealing between the two halves of the casing. 05/10