After replacing the rubber washers in the brass gerber valves (handles) and reassembling the handles, the water flow to the shower head was greatly reduced. The local lumber store didn't have all the washers and suggested I push plumbing string around the stem to stop the water from leak from the handle. This stopped the leak, but the shower flow is weak. All other faucets function normally. I completely removed the shower head and the flow did not increase. There is not a low flow device on this shower.

Where exactly did you place the string? It should not be used as a seat, it goes above the seat on the stem just below the ring. Graphite string stops leaks along the stem shaft not at the seat. If you tried it as a seat it is jambed into the valve body like a plug. Shut off the water and look inside the body with a flashlight and pick out the string with a tweezer.

This is where I placed the string on the handle side of the stem and pushed it in with a screwdriver.

http://i186.photobucket.com/albums/x...C_0002_adj.jpg

I'm not familiar with this exact valve but your screw driver & string idea may work. The valves I've used the string would be between the seat and the brass nut. If you close that valve you'll be able to see inside where I'm talking. In any case the string is not causiung your poor flow rate. So lets go after the original problem. Obviously your main is shut off so have a friend open and close it quickly while you hold a towel or bucket over the open valve body. How was the flow? Maybe you caught some dbris in the towel that was blocking the line.

I'll give that a try. Does the flow have anything to do with the position of the valves when installed -open or closed?

Install your type of valve in it's open position. If you closed the valve and then installed it there is a good chance of crushing the washe.