2004 Civic LX, brake fluid leaking inside

My daughter complained a few weeks ago about something leaking onto the brake pedal. I suspected brake fluid. We brought it into a local Firestone, who replaced the master cylinder. That was two weeks ago, but she tells me it’s still leaking.

They did check the brake booster and said it was fine. The brakes feel fine and it’s not losing much fluid. Any ideas? Obviously, we’re planning to bring it back.

2004 Civic LX sedan, ~315k.