Brake pedal jumping while driving

Model LOGTH2448T

Trying to revive this mower back to full health. When driving or sitting the brake pedal jumps back and forth making all kinds of noise, louder than the engine.

Video of what it's doing: https://youtu.be/oTKP3jKYWGQ

Could this just be the drive belt? Possible adjustment? I'm guessing some of the pulley's need changed. The hyrdo transmission seems strong.

Found the problem. One of the pulley's is chewed up.

And by virtue of the pulley being bad, the belt will be bad as well
Good luck