I've had to extend the viewfinder on my 700 to the maximum position. And now i can't slide it back!
Any clues?

The JVC GY-HM7XX series use the new viewfinder. Held in place by a spring loaded bearing fitting into a small round indentation "hole" on the aluminum rail of the viewfinder. You can with a little force pull the viewfinder off. If you are not able to remove it with a little force its likely the bearing spring is not allowing you to retract the bearing enough to remove the viewfinder. A very tiny amount of a lubricant spray (i.e. WD40) sprayed into the crevice in front of the viewfinder tightening wheel and between the mounting rail. There is actually a small square chennel opening in the centre of the rail that will allow the straw from the spray can.

The vewfinder will easily slide off.

Please note*** to cover the vital components from overspray.