The automatic derailment braking device for railway freight cars is designed to connect the main air duct to the atmosphere in time when a vehicle derails, enabling the train to brake urgently and preventing the derailment accident from escalating. Currently, all new 70-ton class railway freight cars are equipped with this device. 1. Overview of Derailment Brake Valve Disassembly and Repair 1.1 The disassembly and repair process of the derailment brake valve should be carried out in accordance with the provisions of section 3.6 of the "Railway Freight Car Braking Device Repair Rules" (Tieyun [2008] No. 15). 1.2 During disassembly, if the elastic cylindrical pin 3×10 between the valve cover and the valve body cannot be pulled out, the valve body can be clamped on a special arc-shaped clamp and then secured to the bench drill. Use a Φ3mm drill bit to remove the metal from the cylindrical pin, and remove the valve cover. During operation, be careful not to damage the valve cover or valve body hole with the drill bit. 1.3 To remove rust from the inside and outside of the valve body, it can be soaked in a metal rust remover. Heating the rust remover to 65℃ will improve the effect. Use a nylon brush to clean the rust layer from components such as the actuating rod. 2. Composition of the Brake Valve Stem Test System 2.1 Hardware Components The test device is used to test the brake valve stem on a test bench. Compressed air at a pressure of 650–700 kPa is introduced into the brake valve stem and held for 1 minute. No leakage is considered合格 (qualified). The hardware of the test device consists of a test bench, an industrial control computer, a data acquisition and processing module, control components, a power supply, and human-machine interface hardware. The test bench has two test positions. The clamp uses a fast 30mm short-stroke pneumatic device, with compressed air introduced simultaneously. The solenoid valve is controlled by a program to achieve automatic clamping and testing, and the test results are displayed or printed. To achieve rapid positioning and clamping, an O-ring and a positioning pin are installed on the air cylinder base, allowing the brake valve stem flange bolt holes to fall directly into the positioning pin. The valve stem flange sealing surface is sealed when pressed against the O-ring, and the circumferential surface of the brake valve stem flange is pressed against the air cylinder pressure. During the leakage test, a leak detection agent is applied to the outer surface of the valve stem, and manual observation is performed simultaneously with the microcomputer test. After the air cylinder pressure is released, the split-type chucks open, allowing the brake valve stem to be freely removed for the next test. 2.2 The system software uses VC++ to build the main control program, displaying the main control interface for human-machine interaction, realizing all operation functions and serial communication. Functions include: automatic testing, manual (clamping, exhaust), equipment verification, printing, etc. It also displays dynamic pressure curves and outputs test results. 3 Microcomputer Control Scheme The microcomputer control main program framework is built using MFC, establishing dialog box class events and methods. Serial communication uses the Microsoft-provided communication component MSCOMM, and introduces OnTimer and OnComm message loops. The microcomputer and microcontroller define a handshake protocol for timed data transmission and reception. During the test time, the function LeakCheck() compares the received pressure value with the initial value to determine whether the test process is qualified. 4. Microcontroller Measurement and Control System Principle The system employs an 8052 microcontroller with 32K of external RAM. Serial communication uses a MAX232 chip with a baud rate of 9600bit/s, 8 data bits, no parity bit, and 1 stop bit. Sixteen I/O ports are added to control the actuators. The analog-to-digital converter uses the high-precision MAX197 module with 8 independent analog input channels, a range of 0 to +5V, and 12-bit data output. Alternatively, the AD574A module can be used, also with 12-bit data output. The microcontroller assembly program flowchart is shown in Figure 4. The pressure sensor unit uses an RL-PK series diffused silicon pressure transmitter, with a gauge pressure range of 0 to 1MPa, an output of 0 to +5V, and a power supply of 24VDC. The air distribution element uses a 2630 series dual-electro-controlled solenoid directional valve, a two-position four-way valve, with a working pressure of 0.2 to 1MPa and a power supply voltage of DC24V. 5. Test Accuracy Analysis 5.1 Pressure Transmitter Accuracy: Linearity + Hysteresis + Repeatability < ±0.5%FS, Sensitivity 2.5mV/V, Operating Temperature Range -10℃～+50℃, Thermal Zero Point and Thermal Sensitivity Deviation ±0.2%FS/10℃. 5.2 Analog-to-Digital Conversion Accuracy: The MAX197 A/D resolution is 12 bits, with 1/2 LSB linearity, a converted pressure resolution of 0.5KPa, a conversion time of 6μs per analog channel, and an input range of 0～+5V. The overall accuracy meets the test accuracy requirements. 6. Main Performance Characteristics Measurement resolution: 0.5KPa; Voltage: AC220V; Power: 150W; Operating Temperature: -20℃～+65℃; Ambient Humidity: <75%. Utilizing digital testing and microcomputer automatic control, it meets the development requirements of railway freight car component maintenance and testing. User operation is simple, and test results are displayed on the screen, improving the accuracy and reliability of the test. 7. Conclusion This equipment is highly automated, ensuring the quality of maintenance of the automatic derailment brake valve and avoiding rework due to leakage after installation caused by incomplete inspection of the brake valve rod.