Check from rough pumping. If the pumping time is long, it means that there is something wrong with the slide valve pump, or there is air leakage in the front pipeline. If there is nothing wrong with rough pumping, it is about diffusion pump. Generally, it is to clean the pump cavity and replace the pump oil, but sometimes some such problems can't be explained clearly in a few words. Everyone knows how to check the leak, but every time it's a small matter, and it takes a lot of time to find the reason! Good luck!