Sounds like the brakes are working great to me.
Power breaking only works on cars with loosely adjusted rear breaks, or a LOT of power......and honestly, you really don't want to spin the rear tires with the rear breaks applied. That's total abuse of the break system.
What I recommend is simply. Pull the car into burnout position. Hold firmly on the break pedal. Raise the RPMs until the car almost pushes through the breaks, then simultaneously STAB the gas to the floor and release the break. You should get lots of tire smoke. What you are doing is loading the converter and making it flash as high as it can.
