Abstract: The authors introduce the principle of vacuum pressure difference method and its application in testing the cylinder defects of brake chief-pump and analyze the three different seal leakage curves resulted from different defects in the course of manufacturing brake chief-pump's cylinder. According to the shapes of the curves and different leakage values, the authors can easily judge the types, sizes of defects and their positions on the cylinder. The authors also discuss the several critical problems which should be noticed while adopting vacuum pressure difference method in order to ensure the exactness. The experimental bench for applying vacuum pressure difference method was used for in-line testing, of which the exactness is up to over 98%.