Water trickling in copper pipe can make soldering a joint almost impossible because you can't get the fitting hot enough. Stuffing a wad of bread into the pipe will absorb the water and you can get done before it dissolves. But use white bread; whole grains can get caught in valves. Before turning water back on, temporarily remove faucet aerator.
When replacing sections of old galvanized pipe, use two smaller pipe wrenches to avoid twisting and jarring the rest of the pipes. Try to avoid excessive pressure or you may cause more leaks in joints down the line. When leaks develop, always inspect the rest of the piping. An entire section may need replacement, especially if the water pressure is low.