how can I fix my corvec britony II water heater?

My britony water heater's pilot light kept going out making a very distinct clunking noise as if something was tripping out, I could relight the pilot light and it would stay on but when I tried to turn the gas switch to its normal working position it would shut the gas off. I have now purchased a new gas body for the heater and its still doing the same thing does anybody have any ideas as to what could be causing this problem please.