If your system uses springs to pull the jacks up, there's probably little if any pressure in the lines when the jacks are up. If there's no springs, then your system uses pressure to both lower and raise the jacks and there will always be fluid under thousands of pounds of pressure somewhere in the system. If yours use hydraulic pressure to raise the jacks, you need to relieve the pressure before removing any of the lines.
Check the hydraulic control uint for one of two things that will let you bleed off pressure.
1. A "quick flush port" - if you find one, you can loosen it from the housing and relieve the pressure in the system.
2. manual controls on the solenoids that allow you to open them up. You can use this to relieve pressure if there's a path for the fluid to return to the resevoir. As already recommended, disconnecting the pump and cycling the switch a few times should relieve the presssure but i wouldn't guarantee it will.