You need to sand the old pipe clean, so it's shiny. Wipe it dry. IF ANY WATER is finding its way to your joint, your seal will never seal. Try emptying the line from below, and perhaps even blowing air from the bathroom, out to drainpoint below. One trick is to 'stuff' a rolled up ball of white bread (not crust) into the old pipe, and push it about 3' down. If water were to be drawn towards the heated pipe, the bread will absorbe the water, expand, and prevent the water from making its way to the heated part of the pipe. Then, when you're finished, the bread will be pushed out and disolve under pressure.
Also, I flux both the old pipe AND the inside of the joint/fitting.
You're suppose to heat the joint, NOT the old pipe. You want the heat from the joint to 'transfer' to the old pipe, not the other way around.
Roll yourself a 'hook-shaped' piece of solder. Evenly heat around the joint. DON'T try to melt the solder directly. Let the heat from the pipe melt the solder. Heat the pipe, then touch the pipe w/the solder. Heat the pipe, then touch. Repeat until the solder starts to melt on the pipe. Make sure you run the solder all the way AROUND the pipe! That's the purpose of making the hook in the hard solder. (The hook allows you to get to the back side of the joint.)
Get a book and good luck! Father's Day is coming. My best to ya and hope this helps.
Jay J -Moderator
PS: God BLess America!