Yoou experience exactly the reason recovery tanks were instituted. It is called heat soak and pressure build up. Without the coolant moving and the engine heat after shut down saturated the coolant in the engine to the boiling point and built pressure, releasing the pressure cap and allowing coolant out on the ground. A recovery tank of any kind would solve this and the use of the proper cap would allow the coolant to return to the radiator after cool down. The recovery hose has to go to the bottom of the tank so that the end is in the bottom of the recovery tank and able to syphone teh coolant.