The article dedicated to detect the position of the mine hold inside the brake master cylinder block and the existence of sand holes or not. Due to the cylinder block compensation hole deviating from the cylinder center axis, so there may be a large error in the machining process, and because the shape of brake cylinder is complex thus how to accurately detect compensation hole position was a big difficulty. Find the zero line by displacement sensors and detection by CCD, process the date by computer, every part work around efficient rotary table.