Not sure of the problem (other than your PVB is in three pieces) without pictures. Was the system shut down properly in the fall? Did the PVB freeze? Did the screw come off of the top of the PVB? If the PVB is in three pieces then you will have to replace it. If you are unsure, or if you do replace it, you will still have to get it re-tested. You have to test a PVB after every repair, replacement, if you move it, and at least every year by a certified backflow assembly tester.
Zone 5 could have a failed (or stuck) solinoid, or it could just have some debries caught in the diaphram. If you can open it up and clean it, do that, but watch out for small parts and springs. If it's stuck and you can unscrew the solinoid, do that, then tap the plunger lightly with a pen to unjamb the plumger. That can work sometimes depending on the make.
Let us know what happens, or show us some pictures.