Originally Posted by Loren

The air outside the sealed brake system is at ~ 14 psi. The brake pressure inside the system under braking is about 10,000 psi. It's not unreasonable to see a slight leak of fluid, but no sucking of air in just the right circumstance.

Typically, the air in a caliper that's been bled properly comes from boiling the brake fluid and releasing trapped moisture, not from air being sucked in.