Tre Mercati "Marina" Shower valve too stiff

[ysowl]

I have an obsolete Tre Mercati "Marina" shower valve. Replacement cartridges not available.
The inner, temperature control knob works OK but the outer, flow control, is a bit too stiff for my partner's arthritic wrist to turn off easily.
The manual says to soak the cartridge in vinegar.
I can't have the shower out of operation for longer than 8 hrs (have to drain CW tank) so I'm wondering if anyone has tried this and if it worked.
All experience welcome!

 

[steadyon]

1. You could try "bunging" the cold water storage cistern to save draining down. Something like Toolstation 31768
2. You could try something like Kilrock descaling agent - its much quicker than vinegar, but needs good ventilation.

 

[ysowl]

1. You could try "bunging" the cold water storage cistern to save draining down. Something like Toolstation 31768
2. You could try something like Kilrock descaling agent - its much quicker than vinegar, but needs good ventilation.

Thanks @steadyon.
Bunging isn't an option sadly. Kilrock is a good thought, especially if I can find where I put it 3 years ago.
Cheers!