I work in plumbing supply...I'd say just get a new valve as well. Tracking down Pegasus parts is a task even Quixote wouldn't take on! Pegasus is also pretty low quality...you're going to end up replacing it anyway, save yourself the hassle and money of looking for the part. Go to a wholesale plumbing supplier (not Home Depot or Lowe's). Get a Delta or Moen. (Although Delta is fine at home centers). You'll be better off, and if you ever have to find replacement parts again it will be much much easier

Cleaning out the valve body and lubricating the new cartridge can help prevent premature failure, as can flushing out the valve body with the cartridge removed, though this part can create a mess if not thought through first.

Yesterday Am I gave up and called a plumber. Be fore I saw Mule Meat's post. What grade would you give this job? A,B,C,D, F? Told plumber the stem broke off and the cartridge was jammed. He said he couldn't do remove it either, I was tired of being without water so I ok'd replacement. He removed the copper piping and used PEX, and shark bites. The wood at the bottom is to keep the faucet in place, without it there is a 3/4" gap between the shower tile and the faucet. It still wiggles a bit. When I turn on the new single handle fixture, it only turns 1/4 of the way rather than the typical 1/2 way. The water pressure is less than 1/2 of what it used to be. It looks like a whole lot of connections for possible failure. The area is tight, but can't the hot connection be done like the cold ? Couldn't the pex from the faucet be shortened so less wood (or bracing) could be used? Thanks for reading April

Thank you all for your replies! No changes made to the plumbing; a plumber assessed that it looked fine before the contractor installed the cement backer. (Two layers of Redgard waterproofing after that.) We don’t have the original anymore. There was mold in the walls and everything was thrown out. Unfortunately, the fixtures arrived after the contractor installed the walls, so we can’t install the valve that came with the set we bought. I’ll try taking the picture to a plumbing supply store and see if we can find a match.