The way any overflow recovery system that I used worked this way. You fill the radiator to the top and add coolant to the reservoir. Put on the correct cap with the rubber seal on the bottom. Then when the car heats up the overflow cap opens and the coolant goes into the reservoir. When the car cools down the coolant is sucked back into the radiator. Are you using a different type setup? Is a catch can set up different. Please fill me in.