I am new to your site, and have searched extensively for an answer to my situation, but no luck so far. My shower valve, single handle Moentrol, over the last 6-9 months has slowly lost pressure from the shower head. So I started with cleaning the shower head, no change. I then moved to my tankless water heater, replaced the water filter and decalcified the tankless, no change. Yesterday I pulled the valve apart and did the following with no improvement;
1. flushed cartridge body then inserted a new cartridge
2. flushed balancing spool body, then cleaned balancing spool and replaced
3. flushed both stop-check valves and replaced
4. rechecked the shower head
The reduction in pressure has been very gradual over several months and all the above seems to have made no improvement in my situation. Any advice will be greatly appreciated.

you ran the water with the cartridge removed ? how was the flow?

What happens if you run the water WITHOUT the shower head. It, and the balancing spool are the two areas that can affect the pressure.

The pressure was best with the balancing spool removed, and the stop-check valves removed. These two tests were done independently. The pressure was not as good with the cartridge and shower head removed. These two tests were also done independently. Moen is sending me a new balancing spool, and I hope that fixes it but I have my doubts because I took the balancing spool out of another shower that I have and nothing improved. The 2nd shower and balancing spool are working perfectly. Hope this helps and thanks again for your advise.

If your valve was installed like 99% of them are, there is a plug or capped opening on the bottom of it. When the water leaves the cartridge it goes DOWN to that plug and then up to the shower. SOMETIMES, debris builds up against that plug and restricts the water flow up to the shower head. This is more common with a Kohler valve, but can happen with any that are converted to shower only operation.

Did you run this by Moen. Explain each thing that you did and see if their engineering department has a solution. What is the house water pressure and are you on a well or city metered water?

Sorry it took so long to get back in touch with a follow up, I was waiting for the new balancing spool to arrive and it finally showed up today. To first answer the 2 previous posts, the Moen Moentrol does not have a plug or cap on the bottom. I do not know the actual pressure, but it is on city metered water and there is great pressure at all other outlets throughout the house. So today I took the old balancing spool out, cleaned the inside of the balancing spool orifice as recommended by Moen, flushed the balancing spool orifice, inserted the new balancing spool, and then checked the pressure coming from the shower head, NO change. Removed the shower head and rechecked the pressure from the shower neck, NO change. Any other advise is greatly appreciated. Thanks