Okay, so did my first lengthy drive after replacing both rear wheel cylinders, and after a while noticed that the brakes were momentarily losing pressure. Press the pedal and they start to bite and all feels normal, then at a random moment they'll lose pressure for just a fraction of a second and the pedal will drop slightly before they bite again. You can hear a sort of clunk noise when it happens, which I guess is the pistons releasing and immediately applying again.
This doesn't happen with every use of the pedal, but it's probably about 75-80% of the time. It's most noticeable with gentler brake applications, because you're not pressing the pedal with as much force.
The front calipers are less than a year old and the rear cylinders are brand-new, so I guess it's maybe the master cylinder or the power booster? Or is it just possible that there's a rogue air bubble somewhere in the system, even though I thought I bled the cylinders properly and got a really great pedal?
If an air bubble got up in the line and you did only a few pumps during the bleed out, it might do like you discribed.
I've been wondering that. I've got a set of speed bleeders in the mail, so I'll do a complete brake fluid flush when they get here. Weirdly, though, it only seems to happen when the Jeep is moving, although it's definitely not ABS-related. I rolled the front wheels up against the curb and kept pressing the pedal while idling in D, and the pressure loss didn't happen.
FWIW, finally got around to fixing this problem. I installed the speed bleeders and ran a couple of bottles of brake fluid through the system by pumping the pedal about 30 times at each corner. Now the brakes work completely normally. It must have been a trapped air bubble or similar causing intermittent problems.