Based on the system architecture of a certain civil aircraft,according to the requirements of the reserved energy in article (h) of article 25.735 of CCAR25 of the airworthiness standard for transport aircraft,when the emergency brake system uses the brake control valve as the actuator,the output pressure is proportional to the pilots pedal command to the brakes.The internal leakage of the flapper-nozzle pressure servo valve core component of the emergency system will directly affect the selection of the reserve energy accumulator volume.According to the SAE AIR5372A standards,a pressure servo valve with a single nozzle valve structure with low internal leakage has been used in foreign civil aircraft brake systems.The traditional four-way structure and the latest single nozzle valve structure were studied deeply.The advantages of the single nozzle valve in internal leakage,slide valve level driving force,slide valve level parts manufacturing and assembly process improvement were compared with the traditional domestic valve.It is improved in the performance stability,anti-pollution ability and reliability of the whole product.