I have a grapple and had the same problem. Here is the secret: I had left the grapple open when I quick detached it so that it would stand upright just so and I could easily hook it when I wanted to reattach. (can you see where this is going?)
Indeed, I easily picked it up with the quick attach and went to hook up the hydraulics. No way I could relieve the pressure. I even loosened the fitting slightly. That's not the answer. Here's the answer, because here is the problem: The grapple was open and wanted to close itself (thanks to Mr. Gravity). That generated the hydraulic "pressure". Lose a little fluid, still wants to close, still generates pressure. etc.
My (helpful) dealer diagnosed the problem and here was his solution: go up against a tree and push to "open" the grapple. Once I understood, I had an easier solution: took my hydraulic jack and pushed up to "open" the grapple and equalized the pressure. One coupler slid in as slick as **** through a goose. Then, I released the hydraulic jack and now gravity helped me by easing the other (direction) coupler.
Of course, the best answer is to only disconnect the hydraulics when the grapple is completely shut and gravity is not trying to push in the hydraulic cylinder. If there is some build up of pressure after disconnect (hot sun, black hoses, liquid expansion) the same trick should work. In fact, I plan to get a small "bottle" jack for just this purpose (I was able to get to my big hydraulic floor jack, the first time).
An alternative (besides the tree trick) would be a stout piece of firewood properly placed and downpressure or un-curl, depending.
