During the lifting process of the hydraulic system of the dump truck with air pressure, the air pressure passes through the inlet port P of the limit valve (connected to the control valve) and port A (connected to the lifting port of the hydraulic valve), causing the carriage to lift. 

When the dump truck is lifted to a certain height, the lifting cylinder or other transmission device touches the distance adjustment limit rod of the limit valve, and the passage between port P and port A is cut off. Port A is released into the atmosphere, and the hydraulic system has no air pressure input. The carriage stops at a certain position, It plays a limiting role.