I worked on my car. It's my second car a 2004 GM Pontiac Grand AM.
If your unlucky like me check if you have green break fluid. That's bad.
Copper brake lines starting to corrode? That's never fun. What's even less fun is having one explode while you try to apply the brakes...lol...happened to me twice on my old car, on 2 different lines.
The other thing that can cause that is extremely unlikely for your case. Sometimes factories use a green grease in the master cylinder when the car is built, but that would mean that you've never replaced the brake fluid, ever. ..so I would hope that's not the case with your level of automotive common sense...lol