I always replace the cap first. It is cheap enough to eliminate that first. Then look for bubbles in the water, this is easiest to see if the thermostat is in and closed. If it bubbles with no radiator flow, you have a cracked head/block or blown headgasket, Since you are losing water, you probably do have a leak somewhere, check the freeze plugs, I had a new set corrode out on an IHC v8 once in 8 months. Without the thermostat in, or with it open, make sure you are getting fluid flow in the radiator, if you see no flow, the water pump impeller could be loose or eaten up. If all this checks out, have your radiator rodded out. If you have ever used any stop leak in it, they tend to restrict a bit.
