If you really must brief (which I advise you not to do), I would just do facts (2 sentence AT MOST) & rule & policy. Just enough to refresh your mind if you glance at it, but not enough to actually educate yourself on a topic. It's a waste of time!
This is true for some people and not true for others. If you find it's helpful, do it. If you don't find it helpful (and aren't required to do it), don't do it. It seems like most law students progress from briefing in full to book briefing to not briefing at all, at different rates and stopping at different places. That doesn't mean, however, that briefing doesn't serve some purpose in the beginning for many students. A lot of people find it helpful in learning how to identify what's important in the readings and how to organize that information, both on paper and mentally.
If you are having difficulty keeping up with the reading for one class over the summer, I'd take some time to seriously reflect on your ability to keep pace with a full work-load once the fall semester begins, and whether or not law school is right for you. Remember, law school ain't free (for most people anyways); if you foresee yourself falling behind and failing, you'll be out quite a bit of tuition money.