Replacing flat seal in Kohler Cimarron

I am trying to replace a flat seal #1059291 at the bottom of the canister flush valve. The instructions say to remove the stop by rotating it a quarter turn counterclockwise to unlock, then pull free. My problem is that the stop won't rotate or pull free, and I am afraid of breaking the guide if I force the stop to rotate. Anyone with experience on this problem?

Just try a little harder. The keeper will spin off and then you can rotate CC out the canister.

I wonder if pressing down a bit before rotating the 1/4 turn would help. I have never touched one of these things.