Thought you guys might enjoy a follow up and follow up question. So this thing (call it the outer portion of the cartridge) was stuck in the valve but good! I figured the rubber on the side had swollen and were not allowing it to slide out. So, what I did was take a pair of long-nosed vise grips and clamped on to the flange at the top right on the notch. After pulling and slipping off countless times, I was able to flex the flange forward enough to get a small screwdriver behind it. I though I might be able to push through the plastic and give me something substantial to pull on. I grabbed my friendly 16 oz persuader and gave the screwdriver a relatively light tap. The entire front facing surface broke off (the part the nut seats on) exposing the stuck outer cylinder with plenty of room to grab on to it with small vise grips. Another 15 minutes of vigorous pulling and the plastic part of the core came out, but the rubber portion right over the holes in the valve body were still in there! I had to go in with long-nosed pliers and literally peel them out of the valve body. They were somehow corroded into place. An important detail to the follow up question is that I did not have to stick anything hook-like or metal into the valve body. I was very careful when removing the rubber so as not to scratch the surfaces.
Putting it back together now, I got the replacement cartridge, put a little plumbers grease on the on the O-rings and rubber portions, and slipped it back in. It went in smooth and easy, seating completely. I put the retaining nut back on, attached the knob and tested. Everything seems to work, hot and cold, but when I shut it off, I have a fairly rapid drip, in either hot or cold positions. There is also a minor vibration in the cold side (maybe the shut off valve?) when turned all the way cold. Since no one is taking ice cold showers, that's not much on an issue. But I sure would like to get rid of that drip before I silicon seal the faceplate.
Is it the seat at the very base of the stem that causing the leak? Do I need another cartridge maybe?
Thanks!