The hose from the radiator needs to be under the coolant level and the tank itself needs to be vented to allow the coolant to replace the air in the tank and pull it back without restriction.
That tank I made was actually a thermos and the hose from the radiator goes into the bottom of the inner liner with fittings and rubber washers for sealing to the tank.
The vent is on the inside of the tank at the top and it vents between the layers and out the bottom of the outer tank shell when it needs to vent or actually overflow the tank itself.
