Gary Im not stuck yet. I asked if you could provide some pictures of the wet brick. If I can see the problem Im sure I can provide you with more information.
I also provided you with other alternatives to tearing out the brick.
When it comes down to it, Gary you are going to have to tear out something. This ptroblem is not going to be one that you can turn a pipe, ot tighten a packing glan.
You have a leak in your plumbing somewhere. That we know. We also know that the flow test eliminated the faucets. That leaves you with the drain pipes, or the shower head feed pipe. Unless you can see them you will not be able to confirm which is the problem. I presume that the drain pipe is embedded in a concrete slab. If that is the case there is a way to test to see if it is the problem. Before you get into the shower pour a bottle of food coloering into the drain. Do that every time you shower. In a couple days you should see the coloring coming from the brick.
If you dont then the problem is the shower head feed pipe.
In that case open up the wall and repair it.
In a previous post you typed this "i have checked the shower head and the taps by a plumber". How was the shower head feed pipe tested? The only way to determne if it is leaking is to open the wall. If the plumber told you that he tested it he lied.
Do the food coloring test, Gary. That will eliminate one last possibility.
P.S. Please do not use the lower 2 ratings. If you do not wish to pay just do nothing (except reply to me). The ratings are used to determine if I remain on the site as an expert.