2001 Chevy Impala Spongy first brake

I'm experiencing excessive travel in the brake pedal. This only happens to the first application of the brake after the car is placed in drive. Going directly from reverse-to-drive does not reproduce this problem, but park-to-drive does.

This may not be related, but there is also a brief single clunk felt when driving in reverse - only happens one time and at a distance equal to one tire revolution.

I noticed this happened after backing out of the garage a few feet, the brakes "clunk", I back out a few more feet and then I put it in drive. Everything is dandy until the end of the driveway, when I go use the brake (first brake in D) and it is soft. Next and subsequent stops exhibit normal brake pedal travel. If I put the car in park and then drive again (without turning engine off), the first brake is soft, and subsequent brake applications are fine. This pattern is consistently reproduced.

First thought is the brake master cylinder, but there is no fluid leak anywhere, and the reservoir level has not changed. No wear or leaks are apparent in any tubing or connectors.

I am really at a loss with this. Any input much appreciated. Thank you!