More than likely either of three scenarios, least likely first:
1) The air was always there but didn't cause a problem until it moved around.
2) Air got in through the caliper upon release of the bake lever.
3) The reservoir was low enough to allow the master to intake some air at an angle.
Well all logical suggestions. But let's see:
1. It had been hundreds of miles since the dealer had flushed and refilled the brake and clutch fluids. And that included a track day. Though Jenn is particularly smooth and may not have pushed it that hard on the track she does use the rear a lot in conjunction with the front, enough to wear the rear pads out in about 7k miles.
2. Bleed screw was tight and tightly capped, but I guess it's still possible.
3. I'd replaced the brake pads after the dealer had done the fluid with a complete service (I didn't realize they were getting low before hand or I would have done them). So reservoir was completely full. But now that you mention it I think the bladder was extended. I don't recall taking fluid out when I pushed the caliper pistons back in and I don't recall it overflowing, but maybe I'm just remembering wrong.
I still think it is weird that it was fine all last year and this just showed itself only after winter storage.
