car is off there is no coolant in its reservoir?

My car's radiator had a leak, took it off the car, got it patched and popped back on, topped the car off with coolant + water. The problem is, when the car is off there's no coolant in its reservoir but when it runs, suddenly Jesus puts it back. I don't know if I should put more coolant while the car is off or if by doing so, I may cause some pressure related damage. Please advise.