Shower located in the upstairs bath is running in the off position. Removed the existing cartridge, the valve is heavily corroded. Installed a new cartridge and pressurized the system, the cartridge is not holding due to wear and tear of the valve. Cut open the backside of the drywall to access the valve. Removed and replaced the valve. Turned on the water and pressurized the system. Purged the air from the lines. The new shower valve is working properly.