Best cocktail

White vinegar tends to have seven percent acetic acid, which I have found works very well on small parts. You have to be patient, it's a very slow process. I have done fuel sender locking rings, they came out after three weeks looking brand new.
It should do well in a corroded piston/cylinder. You might need to pack plumbers putty on the bottom of the piston, to keep the fluid from draining past it.