Got a 40 year old Delta 3-handle setup in a fiberglass tub/shower. For the last 20+ years, the diverter valve has been left permanently in the "up/shower" position and has frozen/corroded that way. Can't turn it at all to run the water through the tub spigot.
Further problem is the slot head of the screw that holds the plastic handle on the diverter valve body started to strip when I tried to remove it .
Not sure how to proceed. Put a big pipe wrench on the diverter handle and try to force it? Grind the screw head off? Call a plumber :-(
There's no shut-off valve to the tub. And access to the pipes behind the shower is extremely difficult- up through the attic and then drop down behind the wall into a very narrow space that is filled eight feet deep with blown-in insulation!