I think the velve is leaking.

It supposed to stop water flows to the center pipe at fixed water level. Now one thing I need to check and feedback is in the center pipe, where the overflowed water go?

I will get back to you later.

All in all, it probably is simpler just change the valve. You velve looks a little complex so it may not be cheap. But you do not need an expensive velve. they are about $10 in home depot and last several years. change it takes only 10 minute.