[Buzz]

The pulling may be the result of two things. If you are nose diving badly under hard braking, and one side of your front suspension is softer than the other, it may upset the geometry enough to cause a pull. This would not occur under lighter braking with less flexing of the suspension. The other is more obvious - If only one side is locking up you will definitely get a pull due to uneven stopping power side to side.
Bob, what if the air in the system is trapped right at the caliper itself? Could that "buffer" the hydraulic line pressure by compressing the air instead of moving the piston? Although - logic seems to dictate that whatever pressure is lost compressing the air would result in an equal drop throughout the entire system....