Why does the self-priming pump stop and then start without water?

Some users reported that in the process of using the self-priming pump, the water can be sucked up after the first filling, but the water can't be sucked up after the pump is stopped and then started. I don't know what the reason is. Let's analyze it below:
There are many kinds of self-priming pumps. The commonly used Self-priming pumps are ZX self-priming pump and ZW Self-priming sewage pump. There is a check valve disc at the inlet and the pump body is large, so this phenomenon is relatively rare. Some self-priming pump products, such as small stainless steel self-priming pump, have no check valve disc at the inlet, and the pump body is small. Some customers do not install the bottom valve at the bottom of the suction pool during the installation process. After stopping the pump, it is easy to backflow the water in the pump body back to the pool, so there is no water in the pump body when starting next time, Therefore, there is a phenomenon that water cannot be sucked after the pump is stopped. In this case, the problem can be solved by installing a bottom valve at the bottom of the water pipe, or installing a check valve at the inlet of the self-priming pump to prevent the water in the pump from flowing back.
Some users do not install a check valve in the vertical upward direction of the outlet pipe. After stopping the pump, the water in the outlet pipe will return to the pump body. If ZX and ZW Self-priming pumps are used, the water will also slowly return to the pool because the check valve disc at the inlet of the self-priming pump is not completely sealed. After stopping the pump for a long time, only part of the water in the pump body is left, and the water can be sucked up only when the pump is restarted.
Some of the outlet pipelines are very short. As a self-priming pump for temporary drainage, almost no valve is installed at the outlet. After the pump is stopped, because the water level in the pool is lower than that of the pump, the bottom valve is usually not installed in the installation of the self-priming pump, and no valve is installed at the outlet, so the air will be sucked into the pump body, and the water will flow back. If a valve is installed at the outlet, close the valve before stopping the pump, and the atmosphere cannot enter the pump body. The negative pressure formed before the self-priming pump will not be damaged, and it will be difficult for the water to flow back, and there will be no failure to suck water after stopping the pump.

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