Sounds like your clutch is engaged. Take the clutch bell off the motor and see if the clutch shoe spring(s) is broken.
Another thing to check, make sure when you are hitting your brakes, you aren't closing the carb completely. You want a small opening when the car is at idle, like .5-1mm. If the carb is being closed, just adjust the idle screw to keep it open that little bit when the brakes are applied.