Water flowing back into overflow tank?

Check the gasket in the overflow tank cap. If it isn't sealed it won't flow back into the system.
Check the tubing that goes to the overflow. If it is cracked, then it will carry coolant to the overflow, but it will leak vacuum and not suck coolant back into the radiator.
If the reservoir tubing is ok check the radiator cap.
You can have the cooling system pressure checked; if it holds replace the cap.
Remember it must be a recycle type cap.

Also check the clamp that secures the hose to the tank. Make sure that it's really tight.

Or the other possibility is blocked hose.

Block is air locked, the thermostat has bleed hole that should let air out. It must be stopped up. You will need to remove the thermostat and clean or loosen temperature sender to let air escape so water will fill block. any plug or hose on the upper part of block can be used as bleeder. Upper radiator hose won't work, because thermostat is below it.

This details will help.

Thanks.

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