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It's normally easier if you pull at a point roughly opposite of the valve stem. I normally sort of dig my fingers on both hands just on either side of that point and kind of roll them in between the cap and the rim to act as a pry rather than trying to just pull, (also prevents me from falling back on my butt when it does let go)
If you look here at a pic, there are 3 bumps that the cap actually locks into, two over be the valve (yellow) and one opposite (red). To install the cap easy, place it so the one side is onto/under the two bumps and then a simple push/pop over top of the single bump and it should snap right on. if you're trying it all over the place it's just rocking and fighting. Removal is just the opposite
Also, if it's super tight or you have banana fingers, a flexible bodyfill applicator (those square nylon scrapers about 3"x4") makes an awesome tool to slip behind the cap and gives your fingers something to pull against rather than your finger nails
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