I have a cap on my radiator and have had one since I built the car in 1996. Never had a problem but the top of the radiator is the highest point in the cooling system as mine doesn't lean back. I have also always ran one of the cheap plastic overflow tanks on it so when it is hot I can see how much water pukes out and then when cold it pulls it back in. Works great for me as I am not concerned with originality and looks as much as handling and functional workings.
Ron
