The leak is likely from faulty plumbing or damaged plumbing... I would think the only way to know for sure is to rip it out and see why it is leaking! If there is a leak at one of the sweat joints... they didn't do the plumbing. If there is a nail or screw, or pipes got flexed or pushed, then they would have done that.

If you have a tenuous solder joint, it can look good, then all of a sudden leak, or just a little jiggling can disrupt it. Well done soldering can withstand a little jiggle now and then!

You can ask my DH- almost all his soldering leaked- but not immediately, at least before we got too far, bad technique when I watched how he did it... he was banned from plumbing and I finished the rest!

My first thought is if it's because of something directly related to what the plumber did, faulty installation, breaking the cartridge (again) or damaging a pipe, than sure, he's responsible. But if it's due to a faulty part or something the plumber had no involvement with, you are responsible. Either way, since you can't see it without opening a wall he's sort of got you over a barrel. It's just my 2 cents but since you know it's most likely due to something he did AND he doesn't sound very responsive to the problem, I wouldn't want him back on the job. I would let him know that you're going to hire a new plumber to fix the leak. And let him know (in writing) that if the new plumber opens the wall and can show that the leak was because of poor workmanship on his part, you are going to hold him responsbile to reimburse you for the bill to fix it. If he refuses to reimburse, I'd definately see him in small claims court. (Be sure to put back up all your conversations with written confirmations and take lots of pictures so if push comes to shove, you're prepared)

I'm knocking on wood as I write this...but we've never had any leaks, and really have no idea, but if it's just a leak and no body's fault, do you think that maybe you can put in a claim so your Home Owners Insurance will cover it? Just a thought.

Thanks for the responses! As a follow up- because our trim kit isn't installed, you can see into the hole. It turns out the delta valve did develop a fracture in it. I didn't think that valves just random fracture (and I know there was no leak before they started work on it!) and they claimed there was no leak before they sealed the walls up.. just seems weird, but I guess it's normal for valves to fail after 8 years?

In any case I offered to cover the cost of plumbing if they would cover the cost of drywall. It will be a VERY cheap drywall job. They can go through the closet (the back wall), cut a hole and put the wall right back where it was once they plumber is finished. They would just need to tape and paint a minimal amount. Hopefully they agree, I think it seems fair! I do know the valve cracked on their watch, so it seems fair they should absorb a small amount of the expense of replacing it.

I thought my struggles were over, but apparently not ! They want to nail me for $700 to do the job (removing the dry wall, replacing the valve, putting everything back how it was). Now I'm wondering how often do Delta brass fixtures just crack? It seems really odd that even our hard water would have caused this. I called Delta and they said the only thing that would crack a fixture is tightening the cartridge holder on too tight. Anyone have experience with this?

The plumber isn't responsible for the existing plumbing, only newly installed items. That's why it's cheap insurance to replace all of the valves as well as the trim. You really should have planned to replace the valves as part of the scope of work from the beginning. Most existing plumbing can be fine for years, until it's opened up to the light and exposed to vibration from other projects. Then it develops a leak. It's your nickel to take care of this problem, and hopefully translate this experience to any future projects. Always replace the important stuff when you have the walls open and have the opportunity.