THe coolant level in the recovery tank needs to be above the overflow hose from the rad. The end of the hose needs to be close to the bottom. When the rad heats up the coolant fills the tank and the air bubbles to the surface. When the rad cools, the coolant is drawn back into the rad. If the hose end is uncovered the rad will draw in air and act just like a rad without a recovery system and liquid level will settle at about 2 inches down from top of rad.
What's your original rad from?
Some old rads aren't made for any pressure at all!