PTO Drive Shaft

I've got a PTO shaft with the yoke that has the collar that slides forward to lock onto the PTO shaft.  There are 4 ball bearings in the yoke that are supposed to release when the collar slides and then lock onto the detente on the PTO shaft after the collar is released.  It worked just fine when I parked the spreader last fall, but I can't get the ball bearings to release and lock onto the PTO this spring.  They rotate with my finger, so they're loose, but they won't release.  I've soaked in in PB blaster, don't think there's any crud in the collar, could be wrong on that though.

I've had to stick the locking end in a bucket with diesel fuel or kerosene and let it soak a day before. Sounds like you might have a bit of rust or grit up under the collar not letting it spring forward enough to push the balls down 

Have one on a post hole auger that was left on the place when I bought it. Does the same if not protected when not in use and also lubed well.
Likely have grit in the assy. and it does not take much to keep the collar from coming forward over the balls.
Second the soaking it in diesel comment mentioned, and keep working it by rotating it while in diesel.