Whu do i lose power when braking

Hello, a voltmeter is not a sufficient way to inspect your vehicle's charging system. Special attention needs to be given to ensure there is proper amperage output as well as reference voltage while the concern is occurring. My guess -- only a guess -- is that your battery is not getting recharged properly, cannot be recharged properly, and/or the alternator may not be putting out enough amperage or have a diode problem internally. You must use a proper tester to find this out, however. You could also have an engine performance issue, if when you depress the brake pedal it causes a vacuum issue at the engine, lowering the idle too much. I recommend a good tech near you evaluate your concern. Good luck.