I have on an old style (early 1900's) cast iron radiator. The air vent valve has a crack in it and it's leaking water. I was told I would need to replace the valve with a new 1/8" valve. I've never drained my system and isolated a radiator. Instead, I'm wondering if I can put some type of a patch on the leaking valve. I tried JB Weld with no luck. Does anyone have a product that would work for this type of leak?