My bet is that the line still has fluid under pressure. Can you put the splitter back on and (with the engine off) run the handle back and forth to release all pressure in the system? If that fails you could unscrew the coupling from the line to vent the pressure.
My experience is that couplers from different manufacturers don't always work well together, and the ones that come with the tractor aren't that great. You might consider just replacing all of them.