That depends entirely on your house. Many basements never flood.
That would be your flush valve in the tank
Install a one way check valve in the line from the house to the sewer.
check at a plumbing supply , for this, [ball check ] i also have had good louck by pushing a Tennis ball in to mine then replacing the top plate. Acts like a stopper,
I am facing this same issue with a basement installation and plan to use a backwater swing check valve, available thru McMaster-Carr on the web.
If carb, the float valve is stuck open. Take air horn off carb and check.
Install a Backwater valve
A dual check valve is similar to a double check valve in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valve.
If you've already double checked to make sure it's turned the right way, you could have a faulty valve,
Converter relief valve ensures high line pressure does not flood/overrun the solenoid.
There is no difference between check valve and NRV, both are same
A check valve allows for flow in one direction and a gate valve is used for positive shut off and a globe valve is used for throttling
Replace the fill valve or throttle down the isolation valve