WARNING: Long story alert
TL;DR version: Im an idiot and should have started with the obvious.
So, my house has a toilet and shower in the basement (counts as a second bathroom, but its not really its own room). Toilet has worked fine since before I bought the house, shower never has.
Tonight, while insulating water lines and improving the efficiency of the house, I figured I had had enough and was going to figure out why it wasnt working.
I found the water line and traced it all over the place, to the shower past the shower, to the toilet and sink, etc. Couldnt find the damn shut off. To do this, i had to tear out some of the ceiling tiles.
Finally, after about giving up, I tried the obvious. CHANGING THE DAMN SHOWER HEAD!!!! Just like that, it worked.


So yeah, I just spent the last hour or so tearing into **** that I didn't need to. I really need some time off......