Service brake accelerator for rail vehicle air brake systems

A service brake accelerator having a line chamber (3) connected to the main air supply line (1) and a reference pressure chamber (6) connected to the line chamber via a nozzle (7). The line chamber is separated from the reference pressure chamber by a diaphragm piston (5) which is biased by a spring (10) against a valve seat (9) projecting into the reference pressure chamber and restricting a nozzle opening (23) to the inner chamber (24) of a second valve seat (20). A second diaphragm piston (17) is biased against this second valve seat by a spring (21). The space (16) surrounding the second valve seat can be connected, partly throttled, via a nozzle (14), to the line chamber by way of a shut-off valve (12, 13) which opens laterally of the reference pressure chamber upon overbalancing pressure upon the first diaphragm piston; this space is always open to atmosphere via a further nozzle (25). The accelerator operates cyclically and has a safe, stable neutral position between its work cycles and rest position.