Replacing the radiator and the thermostat is good preventive maintenance, but that does not address the problem, unless you are having/had overheating issues. If it's leaking at the reservoir cap, I would suspect the reservoir has a crack or the cap is defective, I would replace both. Also check the hoses that connect to the reservoir.
This is how I refill my coolant system (in this order). Fill up the system with a pre-mixed 50/50 (BMW coolant & distilled water). Start the car, turn on the heater and set it to max, open the bleeder screw half way, watch the coolant level in the reservoir and keep adding coolant, keeping the coolant level at the neck of the reservoir. As the engine warms up, you'll see coolant with bubbles flow out of the bleeder screw. Tighten the screw after you get a steady stream of coolant coming out (no bubbles). Make sure again the coolant level in the reservoir is ok, replace the cap and allow the engine to reach normal temp. Turn off the engine and let the car cool down, check the level again, add enough coolant untill the level reaches "max" (indicated by that float stick) make sure you don't over fill. You'll need to check the coolant level again after the car sat overnight. Keep checking the coolant for a couple of days.
