Okay, I`m baffled... (Mazda Protege radiator/overflow tank question)

Apologies ahead of time for what may be a bit of rambling.
I`ve got a 1990 Mazda Protege (151K miles and counting!) which was running fine (relatively speaking). The other day I noticed a bit of a coolant leak from the bottom of the radiator. Since the car is only used for very local trips, I figured that if I kept the overflow tank filled, I shouldn`t run into trouble before I had a chance to fix the leak. I also filled the radiator directly with straight antifreeze and the overflow tank with plain water. The other day, after about 3 miles, the engine temp gauge shot up and was pinned on the high side. After letting things cool down, I found that the radiator was virtually empty but the overflow tank was still full. In fact, the overflow tank looked like it had actually gotten some of the new antifreeze from the radiator. I checked the radiator cap by hand (no pressure tester available) and it seems fine, both the main seal/plunger as well as the negative pressure one (something I never know existed until I checked my factory manual). Also, the hose from the overflow tank to the radiator neck seems fine.
My question (finally) is why isn`t the radiator filling from the overflow tank? Should I just replace the radiator cap as a first step? Should I put in an old Porsche/VW air-cooled engine and not worry about it again Also, as long as I have your attention, assuming the radiator leak is from one of the plastic caps (top and bottom) rather than the aluminum fins, any ideas how to patch it? With all the Permatex products I see listed on carparts.com and elsewhere, there must be something that works.
Thanks in advance for any suggestions. Ken

overflow tank. As the engine cools the coolant contracts creating a vacuum and pulling the fluid back out of the overflow. There is no way for the engine to pull coolant from the overflow with the engine running.
In your case when it cools it just pulls air in from the leak. Also the raidator has to be 100% full to pull coolant from the overflow tank.
Don`t waste time trying to stop the leak with a miracle cure. Either have the radiator repaired correctly or replace it. These engines will not tolerate overheating. Get this fixed as soon as possible or you will end up with a $25 check from the junk yard.