You need to sweat the pipe. First clean the pipe with sand paper on both ends until they are shiny. Apply flux to both ends of the pipe and inside the female ends of the gate valve. Protect any flamable exposed materials near the joint. Heat the pipe at the connection joint with the valve with a propane torch until the solder will melt when you touch it to the connection joint. The flux acts as a sponge to draw the solder into the joint. 3 is a big pipe so be sure that the solder has gone in all the way around the pipe. Good luck.
Every GMC or Dodge truck I have ever owned had too small of a rear drum for it. I don't think there is an easy way to fix the problem besides installing a better rear brake system. You might try changing the master cylinder and proportioning valve, it might fix the problem.
You need a large acetylene b tank with around a number 11 tip for that big of a job. It has to be soldered. I would definitely hire a plumber for that job. The torch,tip,and bottle will cost around $300. roll of solder $11,flux $4. I would charge you around $250 if it was in an exposed location and you supplied the valve and pipe. cheaper and guaranteed if it leaks.
Here's some links to check out about how to solder. The first one has some video clips. Hope it helps.
