Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
Turn the cap until it is loose and then pull the cap straight up. The rubber o-ring at the bottom has a tendency to expand a bit making the cap seem like it will not pull up and out, but it will. Just so you can see the underside of the cap and know what you're working with, CLICK HERE for an image view of the cap. Please remember to leave a rating using the rating system at the top of this page or accept the answer so I may be credited for this Q&A. You can ask follow-up questions any time, even after you rate or accept.
It probably is just that- stuck. Once it's loose, there is nothing holding it in there except that rubber o-ring at the bottom. Give it a good tug. You won't break anything. Make sure the cap is at it's loosest point and then it pulls up and out.
Sometimes pushing downwards on the cap while turning it to its loosest point can help. But again, once at the middle point, it should pull right up.
Not quite sure what to tell you at this point. If the cap will not come off, then it will need to be pryed off, or the reservoir itself replaced (which will come with a new cap).