Thanks for taking the time to read this and try to help me.
Moen shower fixture in a girlfriends 1980's bathroom.
pull out to turn on, push in to turn off, rotate right to left to set temp.
In a perfect world, when a new cartridge is needed, one removes the old cartridge and installs a new one. Under five minutes.
In reality, often, one of the seals on the side of the cartridge has failed and gets hung up on removal and you have to fight with it for a while to get it out. Trick here I found out is to make sure you account for all the seal once it is out ( normally comes out in pieces) before installing the new. I've come across a few of these.
This one had the symptom of leaking from the tub spout. I could push the knob in and stop the leak but when i released the knob it would start leaking again. It felt kinda spongy, or like a spring was pushing the knob out a bit after I release it.
The short version is that after two new cartridges, an after market and an oem, the problem continues. I have turn the water on without the cartridge installed to blow out any thing that might have been in the line. and an odd thing is that when I first turn the water on after installing either cartridge it did not leak until I tested it.
I'd thought I'd check you some Pros to see if there was something that I could do before replacing the fixture.
Before doing anything else call Moen customer service 1-800-289-6636. Moen has had a lifetime guarantee on fixtures since the late 1980's and if they cant help you with the problem they may give you a new fixture. Its worth a phone call. Good Luck