What are some common issues that can arise with old shower valves and how can they be fixed or replaced?

Common issues with old shower valves include leaks, low water pressure, and difficulty adjusting the temperature. These issues can often be fixed by replacing worn-out parts such as washers or O-rings, cleaning out mineral deposits, or adjusting the valve settings. In some cases, it may be necessary to replace the entire valve with a new one to ensure proper functioning.