trying to remove main vent

We just removed about 20 of these last year at our monastery in NJ. We even removed the bushing on the elbow to open it from 3/4" to 1" to maximize venting potential. We used soemthing called "Rustbuster" in a small squeeze bottle that we bought at the local supply house. After several applications and a few hours to let it soak in, we prayed and went at it with large pipe wrenches (24"), one to turn the fitting and the other to counter turn or support the other fittings that need to stay. With some fittings we needed to use a breaker bar on the wrench. It was better if the nipple collapsed because it reduced teh friction on the rusty threads. Good Luck!