Ron,
Tough problem here. Copper pipes don't close down like galvanized pipes, unless you have galvanized pipe somewhere in the mix there. There could be a small amount of galv pipes leading to the tub spout. I have seen that done many times in homes this old. Not smart, but done. If there is plenty of water coming from the valve stems, you may want to consider replacing the shower faucet. I would change it to a single handle, pressure balanced faucet. If you use a Moen faucet, you can use their remodel plate. this allows replacement without doing a tile job on it. http://www.moen.com

I hope this helps. Because your situation is a bit unique, I think everyone was hoping someone else would come up with a solution to your problem.
Terry

: My house is about 25 years old. I get Lake Michigan water. Pipes are all copper. While I get good water PSI in the downstairs shower and in the upstairs sink, I get very low psi from the upstairs shower. I changed the shower head without any luck. The water from the tub spout is also low on PSI and volume. When I take out each of the hot & cold water cartriges, water shoots accross to the back wall. Plenty of PSI & volume. I'm thinking that there is a problem with the mixer. So before I tear apart the wall and replace it, I wanted to know if you agree that the problem is the mixer and if it could possible be back flushed somehow.

: Thanks for your help,

: Ron