delta pressure balance valve

Hi, I'm having an issue with my downstairs shower valve. We have not used the shower for a year or so but the other day ( I think the washer was running) the shower started running. I tried to shut off but nothing worked. I pulled off the handle and the covers to see what was going on. I can twist the knob around and an around but cannot get the shower to shut off. I have a Delta single handle monitor pressure balance valve, model 1300/1400. Any ideas, suggestions?

I'm not a plumber so I am not familiar with how these things fail. I do have two of those valve in my house and I am familiar with what's inside. That valve should have a stop to keep it from going round and round. It is possible that one of the seats has ripped, cracked, or broken. Turn off the supply for that valve, remove the handle, remove any trim that covers the bonnet nut if it is covered, and then remove the bonnet nut. You may need to replace the whole cartridge or just the seats. You wont know what's up until you get a look at the cartridge and seats. The seats are cheap and available at most hardware stores. The cartridge may be a different story a plumbing supply may be your best bet. For big box stores, I think Lowes has the cartridge.

Delta

The pressure balancing unit probably cracked which would cause a leak that could not be stopped. To repair that you need a new cartridge.