If the grande is newer it is more of a pain than the giant.... That being said, I used a screwdriver to open mine. I just slid a (thin-straight edge) screwdriver under the lip and slowly pulled back. I worked my way all around the edge and then pulled with gloved hands. After pumping, you can replace the top by having a friend apply pressure to the top as you slowly use the handle of the screwdriver to re-crimp the top back on. It takes a little while to do it this way, but mine is fine and does not leak at all.
Best of luck!