Thanks, Robert and David.
I was intending not to replace the shutoff valve at all. I've never used it. I cut off the water by tripping the breakers, but maybe I should put a valve there anyway?
I can blow a tiny bit of air
through the valve... an extremely small amount. It's blocked.
Yes, there was a crimped copper band holding the.. PEX?.. tubing over the copper elbow. I melted the tubing with a torch in order to remove it from the copper elbow. I bought some stainless steel crimp rings but got tired of looking for the needed tool to crimp them, and will look for a push fitting/gator byte.
The PEX tubing is 7/8" outside diameter and 0.68" inside diameter to the best of my measuring ability with a good set of electronic calipers. The black pipe (CPVC?) is just barely over 1.0" outside diameter and 7/8" inside diameter.