I have had good success with using Pressure Drain Openers... have used one on both 1 1/2" pvc pipe and 2". They are soft rubber bulbs that hook up to the garden hose, are put into the pipe, and then water is turned on causing them to expand inside the pipe, and then remove clog by trapped pressure. I would think one would easily fit through the open ball valve into the pvc. Worth a try, and very inexpensive... they've never let me down.