[Mr70]

Try using an industrial volatile Toliet Bowl cleaner,not the standard household stuff.
Aim for the heavy duty stuff that has a skull & crossbones on it,and warns of caustic fumes.
Cut it with a lil water first and if that doesn't move it,use it full stength but becareful as it may etch the plating now.
CLR remover might help too,but I find it has the same ingredients as most household Toliet Bowl cleaners anyway.