I don't have any great faith in PB Blaster, but I've freed up many stuck in-line pump plungers. Depends on how bad and what made them get stuck to start with. Had a few Fords with SIMMs that got stuck from lack of oil. Pried loose with a screwdriver, filled the oil box and away they went.
If all depends on what part of the plunger is actually stuck. The CAV pneumatic pump is basically just a box holding a camshaft with pistons riding on it. Note that if things don't work out - all the new parts are available from a place in England. $130 for all new barrels and plungers. $40 for a new governor diaphram.