In the hydraulic system of agricultural machinery, the pressure valve is a kind of component with more applications. The sudden failure of the pressure valve will directly affect the normal operation of the machinery and equipment. This article divides the cause of the failure of the pressure valve and gives an example to introduce the test method of the pressure valve. 1. Common causes of failure (1) Hydraulic spool and valve body constantly wear and tear in use, making the parts size, shape and surface quality changes. (2) Balance spring, valve core and valve seat in the hydraulic valve work under variable load for a long time, resulting in fatigue and cracks, causing the spring, broken valve seat sealing surface flaking and damage. (3) The remaining residual stress in the process of hydraulic valve parts processing and the external load stress in the process of using, exceeding the yield strength of the part material, the parts are deformed. (4) hydraulic oil mixed with excessive water or acidic substances, the use of a long time, the corrosion of parts in the hydraulic valve, so that it should lose the accuracy. (5) unbalanced radial force between the valve core and the valve sleeve, and finally the valve core is pressed against the inner wall of the valve sleeve to be jammed. (6) When the hydraulic valve is manufactured or repaired, the specified technical requirements are not met; or the parts are not kept in good condition, rust occurs, dirt is mixed in, and the like. 2. Examples of test methods Pressure relief valve, relief valve and sequence valve, etc., this article to illustrate the pressure relief valve test items and test methods. Pressure valve test can be carried out on a dedicated test bench, but also in a comprehensive test bench. (1) pressure regulating range and pressure stability test. Adjust the test valve adjustment hand wheel, from full open to fully closed, and then to fully open, from the pressure gauge to see the pressure rise and fall as well as the pressure regulating range. Pressure gauge pointer should be steady rise and fall, pressure regulating range should meet the provisions of the pressure regulating range, the maximum pressure, the pressure pulsation value shall not exceed the specified value. (2) the amount of leakage test. Adjust the hand wheel is adjusted by the valve to the fully closed position, adjust the overflow relief valve to be the highest pressure of the valve under test, and then adjust the test valve adjustment hand wheel, so that the test valve overflow leakage measurement. (3) Unloading pressure test. The remote control port of the relief valve connected to the tank so that the test valve through the test flow, measuring the pressure relief valve inlet and outlet pressure, the difference is the test valve unloading pressure. (4) pressure loss test. Adjust the test valve adjustment hand wheel to completely loosen, and make the test valve through the test flow, pressure gauges measure the inlet and outlet port pressure relief valve, the difference is the pressure loss of the valve under test. (5) opening and closing characteristics of the test. Close the overflow relief valve, adjust the tested valve to the maximum pressure, adjust the overflow relief valve to depressurize the system gradually, and measure the overflow volume of the tested valve when it drops to the closed pressure of the tested valve. Adjust the overflow relief valve, the test valve does not overflow began to make the system gradually boost pressure, when it rose to the test valve opening pressure, measured by the valve overflow.