Low water pressure from moen shower faucet - Moen 1 Handle Lever Shower Faucet With POSI Valve NI

I'm slightly in the dark on this because I have no idea which model moen shower valve do you have? Here's a shot in the dark....single lever anti-scald tub and shower valves, moen brand in fact, prevent scalding by a means of a cog or two, (a cog is a flat metal piece, with an arm sticking out on a 90 degree angle, and has a star shaped cut out in it's center. That star shaped cut out fits perfectly over and on to the center stem of the valve. These cogs are located under the handle. These two cogs are what limit the handle from opening completely counter clockwise. When the handle is at it's most open counter clockwise position....the port that allows hot water to enter the valve and come out of the shower head is at it most open position. Those two cogs are what prevents the hot port from being fully open by means of the 90 degree arms sticking out and under the handle. The handle has two flat surfaced walls that catch on those arms sticking out from those cogs under the handle. You have to position the cogs on the spindle, (valve stem), in a position that will stop the handle short of full counter clockwise to prevent the shower valve from opening enough to allow enough hot water to pass through the port to possibly scald a person while showering. It's a real tricky process setting those cogs to a position so that when your valve stops it's counter clockwise rotation, (which is how the valve it turned on), it stops in a position that only allows enough hot water to flow that mixes to be a nice warm shower water temperature. You see, with the hot water flow limited, there is little chance you will ever be burned by your shower water (thus...anti scald mode is in operation).....
Now here's the kicker.....When soldering the connections on your shower valve, (during the installation process), the installer must remove the cartridge from the valve body to prevent melting it with his torch while soldering the joints. When that cartridge is removed, those two cogs hang loose and slide off the spline. And when they are removed, it is quite a chore to place them back into the position they are intended to be placed in. and..WHEN THE COGS ARE RE-PLACED IN A CERTAIN (WRONG)POSITION, IT FORCES THE HANDLE IN A POSITION THAT IS WRONG, AND WON'T ALLOW THE HANDLE TO REST IN THE OFF POSITION. The cog arms are preventing the handle from rotating to a completely closed position. Which causes exactly what I think you are experiencing. When the main water valve is turned back on....you can't stop the water from flowing through your newly installed shower valve. Those cogs...in the wrong position on the spline....will not allow the handle to spin to the correct point of the off position.
You have to remove your handle, slide the two cogs off of the spline, then re-place the handle on the spline and turn it until the water stops flowing out of the shower head....now pull the handle back off....look at the underside of the handle....you will see a couple of walls within the open underside. those walls are what the arms on the cogs hit to stop the rotation of the handle....Now hold your handle in front of yourself and face the shower valve....now position the lever on the handle straight down....now lean the handle back to view the positions of those two walls on the underside of the handle.....now imagine those two arms on the cogs sticking out in the area between those to walls ....with one arm positioned to stop the handle in the position with the lever straight down....then imagine the other arm sticking out in a position somewhere around 11 o'clock, (if you look at it like a clock), .....so now the first arm is around 6 o'clock and the other at 11.....now slide the handle onto the spline, with the lever down .....then test the valve by turning counter clockwise to turn it on. ....turn it until it is stopped by the cog arm....now feel your water temperature....if it's good...then turn the handle clockwise to the off position....is it all the way off? if not, pull the handle off...adjust the 6 o'clock cog arm back toward the 7 o'clock position, just one spline ....replace the handle and try again.....that should fix the off position....if you need the water a little warmer, move the 11 o'clock cog arm toward the 10 o'clock position a position or two....to get a cooler mix on your water....move the 11 o'clock arm the opposite direction.... WHEN YOU HAVE FINISHED RE-CALIBRATING THE VALVE....SECURE THE HANDLE AND COVER BUTTON BACK IN PLACE AND CALL IT GOOD...IF YOU DIDN'T DROP THE HANDLE SCREW OR ONE OF THE COGS DOWN THE DRAIN......OOPS...SHOULD HAVE MENTIONED TO PLACE A TOWEL OVER THE DRAIN WHEN YOU DIS-ASSEMBLE YOUR SHOWER VALVE...I hope your not corn fused by my lengthy instructions....and I hope you understood the terms "cog"(the two removable metallic pieces that slide on and off the center spline), -"spline"(the metallic stem sticking out of the center of the valve, it has gears all around it) -"handle lever"(the portion of the handle that protrudes outward, away from the center) -"walls on the underside of the handle"(the two flat pieces molded into the underside of the handle that act as stops by not being able to spin past the protruding arms of the cogs)...Good luck my friend and remember to always work safely....