I am trying to replace a Moen shower stem. I've removed everything off the turn handle up to the pin that locks down the shower stem. The stem won't pull straight out though. I purchased the special stem removal tool, but even with that screwed in I can't get the shower stem to turn the 90 degrees. The tool spins past the two metal ears that protrude from the shower stem. The stem appears to be heavily blocked up - it is the original stem from the mid 1970s and it has never been changed, and the shower hasn't been used in nearly 7 years. It is probably a ton of mineral build up because the water is very hard. The wall pipes are copper but I believe the piece that holds the shower stem is brass. How can I get the shower stem loose? Can I use CLR or Lime Away? Does that risk ruining the brass? Are there any tips for getting rid of the build up to get the shower stem to pull out or turn 90 degrees counter clockwise to then pull out?