Any of several factors could cause your shower mixing valve to lose pressure, continue dripping or even running when you try to shut it off. Find out below about these three things to check when your shower mixing valve will not shut off.
1. Clog in Mixing Valve Reduces Hot Water Pressure
With a single handle mixing faucet, if the water pressure drops off when you turn the handle toward hot, the mixing valve may be clogged. You will need to take the faucet handle off and disassemble the mixing valve to find the blockage in the valve ball or stem. Clear the clog and reinstall the mixing valve.
2. Worn Mixing Valve Washer
The mixing valve washer may be split, cracked or worn. Try replacing the washer to see if this solves the problem. If after replacing the washer, the faucet keeps dripping, take apart the mixing valve to check for wear in the valve seat or screw fastening. Replace the worn parts and reinstall.
3. Check the Mixing Valve Insert
The valve insert itself may be plastic in some models. It may have cracked due to excess twisting pressure, or pipes freezing. Replace the mixing valve insert with a brass piece if one is available for your valve.