I don't have a definitive answer for this one, I can only tell you what types of things could cause a random pull L&R when breaking... I don't know the specifics, so I'm taking a guess... It seems they have checked the ABS and the calipers, but have they checked whether the rotors are warped or if they seat "true" on the hub ? It could be warped rotors or the hubs do not spin on a perfect plane.

It could also be a hydraulic problem whereby the fluid is not flowing equally to each of the calipers as the pedal is depressed.

My gut feeling is that the rotors are not "true". It is very easy to check if your rotors are warped or not seated correctly. It's a little bit more difficult to test the fluid pressure to each of the calipers.