Shower diverter valve
Most single-handle shower faucets simply turn water on and off; a pull-up diverter valve on the spout determines whether water goes up or down. Some models, however, have a diverter built into the handle.
You may be able to find repair parts in a kit for a specific faucet. If not, standard O-rings and washers may fit. In some cases the seal is made with brass parts and no washers; simple cleaning and light sanding may solve the problem.
A corroded diverter stem or one with a weak spring should be replaced, say the plumbing Littleton experts. A trip to your local Littleton plumbing supply or home improvement store may result in not finding your part stocked, but a learned salesperson will know if a universal part will work. In some cases you should be able to order it. While you are waiting for the part to arrive, wrap duct tape around the handle of the diverter to keep it from pushing too far in,. This will allow you to use the shower while waiting for the replacement part.
Plumber Pro Tip: When pushed in, a washer at the end of the diverter seals the opening so water cannot rise up to the showerhead. A spring inside the diverter counteracts the effect of water pressure so the diverter can be easily pushed and pulled.