My bad, I meant to say Vice grip pliers. Thanks for the recommendation for Koril oil, I'll look for it soon, soak a rag with it and wrap it around the valve. I'm thinking this hasn't been opened in years and needs to be thorughly soaked. I have some replacement bleeders should I manage to get the old one loose. I haven't tested the brakes since I replaced the pressure switch so if the caliper works, I may leave it alone, just keep soaking it and move on to the fronts.
Thanks very much Ronnie!