I had a similar issue that was resolved by adjusting the rear brakes per pinhead's instructions.
I was experiencing an issue with the pedal occasionally / intermittently (maybe once per short drive) going almost to the floor with reduced braking ability. When this happened, the brake sensor light would come on and a single pump of the brake would resolve the issue. I had been driving the truck for several months with no issues, this started immediately after flushing and bleeding the brakes - this was done because the car had been sitting a few years before I owned it and the fluid was looking pretty yellow.
I did four clicks instead of the recommended three, because that is where the wheels started moving freely with some noise. Not only is the issue resolved but it gave me a "oh, THAT is how the brakes are supposed to feel" moment..
Here is a list of other things I tried that did not resolve the issue before adjusting the rear brakes, because I just couldn't get myself to believe that the out of adjustment pads would cause an intermittent issue:
1. Checked for fluid leaks: none
2. Checked for bad bearings throwing the front brake adjustment off: both bearings are fine
3. Checked for air in brake line: re-bled. Thought I saw some bubbles in rear passenger wheel but didn't make a difference.
4. Pulled out the brake booster check valve and tested it: working. Did the start.stop the car and hold the pedal booster testing too. Worked fine.
Next step would have been to replace the master cylinder, followed by the booster. Glad I read through some old threads first.