Adjust the brake lever reach
You can watch a video on YouTube for maintenance.
After riding for a certain distance, it is inevitable that the brake lever will become loose. When you squeeze the brake lever and it doesn't provide enough braking power, you need to tighten your brake lever.
Here is the correct brake lever position:
When you squeeze the brake, there should be a two-finger gap between the brake lever and the handlebar. This indicates that your brake lever is in the correct position and should function well, assuming there are no other malfunctions or wear.
Here is the incorrect brake lever position, which is too loose:
When you squeeze the brake, there is only a one-finger gap between the brake lever and the handlebar. This indicates that you need to adjust your brake lever to improve its braking performance.
Adjustment procedure: Using a multi-purpose Allen wrench, locate the screw hole on the side of the brake lever that needs adjustment. Turn it clockwise until, when squeezing the brake, there is a two-finger gap between the brake lever and the handlebar.
YouTube video link: