Did you know those little rubber seats are usually the first thing to go and not the cartridge itself? I had the same thing happen with a Delta in my last house, and I spent way too long blaming the whole valve before a plumber buddy told me to just swap the seats and springs. Now I keep a tiny zip bag with a spare cartridge and a set of seats taped to the inside of the vanity door, takes up about as much room as a deck of cards. The trick is to write the model number on the bag with a sharpie so you are not squinting at faded numbers in a hardware aisle at 7am. I also keep a spare shower head gasket in there since those fail about as often and cost nothing.