If you have a pressure relief valve that is no longer functioning properly, there are a few things that you can do in order to test it. First, check the manufacturer's instructions in order to see if there are any specific testing procedures that need to be followed. Once you have done this, you can proceed with the following steps:
1) Remove the cover from the pressure relief valve. This will give you access to the internals of the valve.
2) Check all of the moving parts inside the valve for signs of wear or damage. If any damage is found, it is likely that the valve will need to be replaced.
3) Manually operate the lever on the pressure relief valve to ensure that it moves freely and correctly.
4) Closely inspect the seat and disc inside the pressure relief valve. Make sure that there are no cracks or other damage present. If there is damage present, it is likely that water will leak past these points when the valve is open, which could lead to serious problems.