After removing cartridges from shower valves, I often discover that solder, dirt and sand have clogged the inlet of the shower valve, restricting the flow of water. If I turn on the water to flush the valve, water sometimes flows into the wall when the shower valve is fully recessed behind the fiberglass shower.
In order to get the water into the shower, I discovered that an empty paper towel roll will fit into the empty shower valve and allows the flushed water to enter the shower. This saves a lot of mess and avoids any type of possible water damage.