Time for a new horn. And I just might be the one to help !
PB Blaster does a good job of loosening stuck slides for me. Much more capable then WD 40. The stuff is pretty smelly though. Being a Martin, I'm sure the laquer is crap, so you don't have to worry about marring the finish. Although, I freed up the slides on a Yamaha with no issues to the laquer.
You can find PB Blaster at most hardware and auto parts stores.