It could be rings or valves, but it sounds like valves to me. Have some one do a "leak down test" on it. You can do it yourself by putting the suspect cyclinder on top of the compression stroke and applying air pressure in through the spark plug hole. If you hear air passing through the breather on the valve cover you have bad rings. If you hear it through the exhaust you have a burned valve. If you have bubbles in the radiator you have a bad head gasket. I made a tool to do this with simple fittings and a compression gauge. Make sure you put it in gear with the E-brake on after you bring the piston to the top of the compression stroke, even a week cyclinder will move the car! If you don't put it in gear the engine will turn over, and at least scare the crap out of you! Good luck,
Tom