George, I had the same problem on the wife's 850. I agree with the responses regarding making sure the calipers are OK and sliding fine. However my problem got down to the control arm bushings. If you can move the control arm with a crow bar/big screw driver it will cause the car to swerve slightly when the brakes are applied. In my case the first application of the brakes would cause the swerve and a quick release plus reapplication of the brakes would set the car straight. I replaced the calipers but the problem persisted until I replaced the control are bushings. What happens is the control arm and therefore the affected wheel moves slightly when the brake is applied. It changes the front end geometry which causes the car to move right or left. The reapplication of the brakes steers the car straight because the shift has already occurred in the front end.

Good luck