The rear or front brake is weak — what to do?

1. Replace the brake fluid.
2. Bleed it: if the lever goes all the way to the bar, there's air in the system. Laying the bike on its left side and pumping the lever about 15 times helps.
3. Fit a spring-steel disc (sold by aftermarket makers).

For the rear, people also fit a 9 mm clutch master cylinder and Polisport levers.

Stark video: rear caliper service, MX 1.2 — https://youtu.be/WPdC1iC7LbQ