https://www.jumpochina.com/reasons-and-solutions-for-bicycle-brakes-not-working.html
1. The brake pad of bicycle brake accessories is too far from the rim
The adjustment method of the front brake of ordinary bicycle brake parts is: use a wrench to loosen the brake nut, lift the front brake fork and the front pull tube up with the left hand, so that the distance between the brake skin and the rim is only 1 mm, and then use the right hand to adjust the front brake. Pull down the short tie rod on the handlebar to make the short tie rod and the front pull tube in a straight line, tighten the brake nut tightly, and release the left hand, the brake shoe will drop by 1-2 mm, so that the brake shoe is about 3 mm away from the rim.