Still could be a proportioning problem though. Basically what's happening is you're partially applying the brakes. Apply the brakes too hard and you can't spin the rear wheels, apply them too lightly and you don't have enough braking force to hold the car in place. What you need is a larger proportioning difference between the front and rear brakes, so you have less force on the rear brakes when you have enough front brake to hold the car.
The stock proportioning valve was designed to stop the car the best, not do the best burnouts. An adjustable proportioning valve would let you dial back the force of the rear brakes, so you could step on the brakes harder and still spin the rear wheels. Now, that being said, you wouldn't stop as well the rest of the time, assuming your car isn't currently set up with too much rear brake bias.
