Shower flowing when valve "off". Blasting when valve "on"

[NYCjstar]

I moved into a house (in New York on hard well water that is really hard on the copper and brass - scale deposits and possibly corrosion). The shower has two shower heads, and 4 body jets. One body jet was dripping a bit: I replaced the Grohe 1/4 turn 3/4" cartridge, and it's fixed (the old valve looked scaly and green). One shower head won't turn off (It has been capped since we moved in). I replaced the grohe cartridge at the handle/valve again expecting to fix it. Now (as before the cartridge was changed), with handle in "off" position, water flows medium-strong. With handle in "on" position, water blasts out like crazy. I could try another cartridge - perhaps this one is defective - but I feel there could be something else going on, since the previous owner couldn't seem to fix it either. I don't want to rip out the tiles on the wall! Any suggestions appreciated.

 

[Terry]

How about some pictures of the valves? 800 pixels or less.

 

[Sponsor]

 

[NYCjstar]

This is a pic of the new cartridge that I put in. Everything is inside the wall, so I can't get an image of it. The old cartridge also looks like this, except the part that was inside/exposed to the water is green (and a little pocky with scale).

 

[NYCjstar]

Turns out it was a defective cartridge/valve that I had purchased (new)! I tried a (second) new one, and when I took the first replacement out, I saw a plastic part of the cartridge had cracked. The new new one worked fine, and all good now.