My brakes work when sitting still, but will not stop bicycle in motion/under power Mongoose Bicycle with front disc brakes and Promax brakes on rear. When I test rear brakes sitting still, the tire is held tight and motionless. However, when riding, the brakes just rub the wheel. They do not even slow the bike down. It's like they are not even there. How do I fix this?

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There are multiple possible reasons:
1. Brake caliper is not adjusted so pads are within 1/8" or so of rim (rim should be straight enough to allow that).
2. Brake pads are old and hardened.
3. Too much friction in brake cable and housing
4. Brake cable housing is too long, allowing too much flex.
5. If brake levers have been replaced wrong levers were installed.

p.a. It is helpful to know whether a problem is new, has developed gradually over time, or has always been there, and if any maintenance or event occurred just before the problem began.
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