Hi. I am trying to replace an old (diverter?) stem valve for a compression faucet in my old apartment (I'm assuming that it needs to be replaced because, though both hot/cold shower valves work fine, it's when I turn the shower valve on, water is still running from the lower faucet along with the shower head.) Though I've removed the handle, but no matter how hard I've tried, I can't seem to get the center valve to budge (counterclockwise). I've pushed and pulled, used not only a 14" cast iron pipe wrench by itself, but used a shower valve socket wrench to leverage it off (even with WD-40 sprayed on the threads).

Because once I remove it, I plan on taking it to Home Depot/Lowe's to get a replacement. Short of calling a plumber, is there a plumbing product or process to help me loosen and remove the stem? Or perhaps a bigger/heavier pipe wrench?

Did you remove the bonnet packing nut, (see image) before you attempted to back the stem and washer assembly out? Most three handle tub/shower valves are made like the image I've put up. If you have removed everything then the stem should back right out.
Let me know, Tom

Good morning. Thanks for responding; I appreciate it. In regards to the bonnet packing nut (930-060 on image), yes, I've tried removing it. But like I mentioned earlier, the whole thing is pretty old, so it's corroded pretty tight. I've sprayed WD-40 on it numerous times, and though I can see the (very) slight breakage between the threads and the copper? Piping, it's on there pretty tight.

I'm thinking about using one of my thin screwdrivers and lightly tapping at the base of the thread/pipe mouth, but I'm afraid that it might do more damage by pushing it tighter into the pipe.

Any suggestions on what to use? I'm also thinking about getting a heavier cast iron pipe wrench for weight/leverage on it (Sadly, I'm a 120 lb female, so I don't have much weight to use as leverage on the pipe wrench to push/pull down.), instead of the 14" that I'm using... or what types of lube I should try besides WD-40?
Thanks!

The bonnet packing nut MUSt be removed before the stem and washer assembly can be backed out. A 14" pipe wrench should be more then enough leverage to remove the nut.
These are brass to brass threads so they won't be rusted in. However, they might be corroded. WD-40 will cut the corrosion. Load the threads up with WD-40 and let it set a while and try again. Good luck, Tom