I have a recently rebuilt 6.2 with banks sidewinder kit. I have replaced the radiator tstat water pump and all hoses and I even by passed the heater core to eliminate it. When my truck gets to temp and flows into the over flow tank it never returns to the radiator. I have no other issues with motor. No smoke white or black no water in the oil no sweet smells. Except for the diesel fuel...haha I am completely lost here

 

The overflow tank accepts the expanding coolant after being heated.

If the rad cap and tank are working properly-the excess coolant should be sucked back into the rad when everything cools down-there can be no leaks in the bottle and the cap must let air back in the tank to take up the space from the coolant going back.

In the old days there were only overflow to the ground because the system does not need the extra coolant-the tanks and double acting rad caps were made to keep the streets cleaner and not poison everything.

 

I found the issue after just running it like a stolen go kart in the driveway when I rev it and build a good bit of pressure the filler neck started to squirting from under the cap so u guess I got a bad Mexico radiator...