Delta recommended the following for a corroded cap assembly:
To remove a corroded Cap Assembly from a single handle bath valve, wrap a cloth around the joint and saturate it with boiling white vinegar. It might take more than one application.
If the above fails, unscrew the plastic ball retainer all the way out, and remove the ball. Apply vinegar to the inside of the Cap Assembly or spray in PB Blaster. If this fails, heat the cap with a propane torch.
I want to replace the seats in a faucet for a friend but, so far, I haven't been able to remove the cap assembly or to remove the plastic ball retainer. She has been applying vinegar to the cap assembly joint as well as to the ball retainer area joint but, if I still can't remove either, what else might work?
I want to avoid replacing the faucet because it would be very inconvenient and time consuming in this situation.