Lately my water bill has gone even water council wrote me a letter as a reminder. I suspected my shower pressure valve is the caused. My shower pressure is not what it used to be (strong enough). There was a problem with the shower wall grouting before but fixed, and lately I notice behind the shower wall, painting are bubbled up and brown stained patches had widen up. But everywhere I touched are not wet... really frustrated. Will the shower pressure valve will cause this problem?

I think you have to look someplace else for leak. If the shower leaked so much that "...water council..." would notice - than you would be flooded within few hours...

Check your sprinklers, water main, leaking toilets, dripping garden hose valves... etc.

If the shower appears to have low pressure than remove shower head, run water w/o the head for a minute or so. Then, submerge shower head in vinegar and leave it there over night. Reinstall in the morning. See if that helped...