On the back side of the valve, there is another screw or hex allen wrench nut as on the front side. It is not easy to access, but if you can work your way to the back, you can open it manually. You must take care so as not to strip the screw/nut on the back. Also, you can loosen the 4 corner nuts/screws slightly to give the slide a little more freedom to open. It will possibly leak a little, so I would put a thick rag below the valve. I have had to do this.
Roy