No, I haven't figured this out yet. But now that several of you have pointed out the AC / vacuum thing, I think that's the root cause. I've only ever noticed this at idle, during the summer months (AC typically is on). I'll pump the brakes with the car off just to check, and I was planning on changing the brake fluid soon anyway.
ras06LGT, it may be less travel than I'm describing. The resolution of my feet is low. Time for a foot mod lol