I've noticed lately that coolant will flow out of my radiator into the reserve tank, but won't get drawn back into the radiator. I'll pop the rad. cap and look inside, and the level is low. If I pour coolant back into the radiator from the reserve tank, it will fill up the radiator, and both will be back at the same level they were before. I'm not losing any coolant at all. I'm thinking that my radiator cap is bad, but I don't want to waste money on something that isn't the problem. What do you guys think?