Bathtub cold water suddenly quit. Hot water faucet still fine. Removed and examined cartridge. All OK. I also had wife stand by as I did a quick turn on and off at the home shut off valve. No water pressure! All other cold water faucets work. There must be somthing plugging the bathtub cold water line. What is best approach to clean out? This is my mother-in-law's home and quite old plumbing.