One of the problems I have when remodeling a kitchen or bathroom occurs when I have to sweat-solder new fittings into an existing copper water-supply line. It is almost impossible to get all of the water out of the pipes, and that leads to poor solder joints.
My solution is to drain the pipes in the usual manner — being sure to open faucets at the highest point in the system — and then use my wetvac to suck the remaining water out of the lines. This procedure requires a vacuum-nozzle reducer to ensure a tight fit and good suction (I use the plastic reducer that connects my orbital sander to its dust bag). Remember to vacuum both the uphill and downhill ends of the line. I find that after about two minutes the pipes are dry and ready to solder.
Peter D. Ellis, Needham, MA