Autocrosser7

Ok. When I'm driving and step on the brakes with the revs still up, the brakes work fine. When I try to step on the brakes when the motor is at idle, the pedal feels really hard and the brakes do very little. If I hold the brakes while it's doing that and rev the motor in neutral, it gets its braking power back. Using the brakes also causes the dash to go dim. Any suggestions?