In known methods for generating a predetermined constant pressure in a test liquid of a test apparatus, the test liquid is acted upon by compressed gas that is pumped continuously by a pump and is vented into the atmosphere once the test procedure is completed. This not only makes for an expensive consumption of compressed gas but also causes fluctuations in the pressurized liquid and generates noise. In this method, the pressurized liquid is pumped into a reservoir by a feed pump and in the process, via a movable wall, positively displaces compressed gas, which can flow back into a compressed gas container via an opened bypass shutoff valve, a bypass line, and a compressed gas shutoff valve. During the procedure of testing the specimen, the compressed gas from the compressed gas reservoir acts upon the test liquid with a constant pressure, regulated via the pressure regulating valve. The method is especially suitable for use in a test apparatus for testing injection valves and Diesel injection nozzles.