The design solves several issues associated with volume and big calipers more than wheel cylinders actually. When CAFE standards for the OE's forced them to design more towards fuel mileage, low drag calipers became the norm. Low drag calipers have square-ish seals that retract the pistons further than necessary to make sure that they don't drag on the rotors which was supposedly good for 1-3% better mileage. Since the pistons have further to travel before making contact with the rotors, the quick take-up MC achieves this with the primary bore. This applies to us also....

I am not sure on the part number change rabbit, I just ordered that bar by that number and I got an 18mm it was $ 85 bucks, but it is good stiff enough 19 would be too much imo. My kid is gone to camp...he runs the pics LOL I don't know how.