Symmons Temptrol cartridge:

Tenant threw us this one today, 20 year old Symmons Temptrol hard to turn. When I arrived handle and splined end of cartridge broken off, so off to the plumbing supply for a T10 cartridge, and a new cap, easy right? So I unscrew the whole thing together and turn the water back on, works great except I cannot turn the water off at the tub, wtf? So I screw the T-10 and compare with the old, the difference being the thick rubber gasket on the old cartridge that mates to the outer (cold?) seat, vs. the new T-10 which has a steel washer with a rubber center. I have heard of the seats being a problem, and actually recalled some years ago? Would a seat change do it? Thanks!

Yes, the seats are probably the problem.

Search Symmons Temptrol here I have posted info on changing seats.

We got it done first thing, I grabbed a complete Temptrol at HD last night, along with the Simmons seat tool, very slick. I removed the cold seat and low and behold, stuck inside was the steel washer from the original cartridge, split and corroded. I changed both seats, the cartridge and cap, good as new. I would never recommend changing one without the other since the seats take a lot of abuse and were fairly beat up.