No Water == No Leak, right?

We have an older shower (no real guess on the age) that was leaking a little bit into the guest room downstairs. I pulled open the dropped ceiling in there, shut off the two valves leading up into the shower, and proceeded to take the bugger apart. While doing so, I found the source of the leak: the hot/cold mixing valve (it's a single-valve/single-knob system with the shower/faucet switch on the tub faucet). So I replaced the rubber o-rings, put things back together, threw open the shut-off valves, and nothing. barely a dribble coming out of the shower. Bathroom sink faucet and water pressure throughout the rest of the house is just fine, but nothing coming out of the shower. So I removed the mixing valve entirely and opened the shutoffs again. Nothing, again.

Thinking that it was a matter of the shutoff valves maybe getting stuck closed, I turned off the house main and popped those off. They're definitely old, but things looked OK, and the rubber rungs on the ends of the stoppers were still attached to the screws. (At least, as best as I could tell). So I put those back in, took out the mixing valve entirely, and turned the house mains back on. No water, even with the mixing valve completely removed, again.

This situation has confounded me. Where could the problem be? Could I have screwed up the mixing valve in my fuddling, causing the backup? Is there just a bunch of old gronk in the pipes someplace? Does my plumbing just hate me?

Any advice would be appreciated.

Also, I turned off the mains, removed the stopper valve stem, and then cracked the mains back on very lightly. The wall and I can confirm that there is in fact water getting *to* the valve, but I can't yet figure out how far past the valve it is going.

If you have a pressure balancing spool, that may be stuck closed. If not, you must have gunked it up.. Try putting a new cartridge in there and see if that fixes it. If you can't clean the old one, you just keep the new one.

I don't understand how a leaky faucet could damage a room downstairs. A leaky supply pipe, yes. A leaky gate valve, yes. A leaky drain yes. A leaky faucet, no. You must have another leak that you haven't figured out yet.

As far as I can tell, I don't have a pressure-balancing spool. Though I've never seen one directly myself. What do I look for in that, and how do I go about un-sticking it if it's there?

What I have looks like this: The valve stem has a flat disc on its back with two holes in it, and that mates to the other side which is a metal plate with three holes in it; two from inlet pipes, one to the pipe going down to the tub spout and up to the shower head.

As far as the possibility of it being gunked-up, what's a good way to go about fixing that? I've got the house water back on and the stop valves both fully open, but that pressure doesn't seem to be doing anything.

As for the initial leak itself: The leaky faucet had managed to drip back behind the wall of the tub itself. The leaking water was definitely coming from the mixing valve area, and it looks like the surface tension of the water was managing to pull it down back sometimes, along the pipes and the edge of the tub itself. It was very inconsistent in how it was dripping, but this is definitely where the water was coming from before.

If I understand you, you turned the service stop valves off in the floor below the shower prior to doing the maintenance. When you turn them back on, you got no water at the shower valve. The washer in the shutoff valves could very well have broken up if it was old and is clogging the shutoff.

A pressure balance valve in the shower, if you have one, would not normally stop all flow, but it could stop most of the hot. If you have one (and depending on the age, you may not), you can usually remove it, clean and/or lubricate it and restore it to new like condition. This is basically a piston that should be in neutral position, allowing both hot and cold water to pass by it. If the pressure on one side or the other drops (someone flushes the toilet for example), the piston moves to adjust the hot volume to maintain approximately the same temperature so you don't get scalded. These are required on new construction or remodels.

You need to isolate exactly where the flow of water stops. If you have water up to your stop valves, but not after, then replace your stop valves, or unscrew the packing nut and remove the guts of the stop valve and replace the rubber bib washer. If you have water up to the cartridge, but nothing gets through the cartridge, then replace the cartridge or remove it and try to clean it.

A picture would help us. You have an unusual problem, and we would like to see exactly what model you have.

Pictures

Here are some pictures of the shower valve in question, two of the shower-side of the mixing valve, one of the valve stem. No discernable brand on the knob, but it looks similar to the Delta brand ones in the harware store.

How about a picture of the shutoff valves?