Problems with your bathtub and shower diverter lead to weakened shower head water pressure. A bathtub or shower diverter literally diverts the flow of water away from the bathtub's spout, redirecting it out through the shower head. What you must do to correct the problem depends on the type of diverter your faucet uses.
Most modern faucets place the diverter valve in the bathtub's spout where you pull up or push down on a small rod to change where the flow of water is directed. The diverter valve and the rod that controls the valve may be located in the front or rear portion of the bathtub's spout. Older faucets use a third handle for the diverter, located in between the hot and cold water control handles.
Diverter Valve Wear
When the diverter valve no longer works, you will notice that water runs out of the bathtub's spout when the diverter is switched to direct all water flow to the shower head. As more water flows out of the bathtub's spout, the water pressure for the shower head diminishes. Once you notice the diverter is failing, take action to restore the shower's full functioning water pressure.
With older diverters, which are located in between the hot and cold water handles, you must replace the diverter's valve stem assembly the same way you would repair the faucet if it leaks water. You must first shut the water in the house off at the main water valve. Remove the set screw that sits in the middle of the diverter's handle, and pull off the handle. Remove any other metal sleeves or retaining nuts, and use a wrench to twist the diverter valve stem out of the faucet. Either replace the wearable parts on the valve stem, or replace the entire valve stem before reassembling the faucet.
You do not need to shut off the water in the house to swap out a worn diverter. For spouts that have a set screw on their underside, you must first remove the screw using an Allen wrench. Once you have removed the screw, you only need to pull the faucet off the water pipe. Spouts that do not have a set screw twist off the water pipe counterclockwise. If you cannot back the spout off the pipe using your hands, use a pipe wrench to turn the spout. Replace the spout with a new one that attaches to the water pipe the same way as the old spout and that has the diverter in the same part of the spout.