Any advice on unthreading old Brass pipe? The brass pipe is in the back with the spigot attached.
Before you assume it's not going to turn, give it a try. You might find that even though the pipe looks like it's in rough shape, it unthreads without too much trouble.
If the pipe won't turn, tapping might break it loose. However, you'll want to be gentle, so you don't break anything. You're not looking to apply extra rotational force, you're looking to knock crud loose from the threads.
If the pipe still doesn't budge, try putting a second pipe wrench in the opposite direction on the outlet of the tee. Then push the handles of the wrenches together.
The second wrench reduces the amount to torque transferred to the fitting, and gives you a bit of extra leverage.
If the pipe is still being stubborn. Soak the joint in penetrating oil, and allow it to soak in for a bit. Then repeat the steps above.
If the pipe is still not moving after repeating the steps above a few times, It's time for more drastic measures.
At this point you'll be at your wits end, so it's time to get destructive.
Be careful when making the cuts, as you don't want to cut into the threads of the fitting. Usually you can break or deform the pipe nub enough, so that it can be removed from the fitting.
Lube it a bunch of times & let it soak for an hour between each lube. Then, heat it up with a torch or candle & go easy with a Pipe or Monkey Wrench. Hopefully it won't snap. But, if it does then you'll still be able to get that with an Internal Pipe Wrench bit.