I have a single handle shower control like the ubiquitous Delta (swings
up & down and side to side). It is leaking and I have the replacement
cartridge, but I cannot get the handle off. The cartridge has a 1/4"
square plastic shaft that sits in a socket in the handle. Held by a set
screw that I have removed (1 only). I'm assuming that the handle is pot
metal and in 18 years of use some corrosion has occurred and bound up
the shaft.

I have put Liquid Wrench in the set screw hole and let it sit, for days.
I have put some heat on the handle, but it's chromed and I was
cautious in the heat's use.

Any ideas?

Thanks,
Bob