Step Bore Master Cylinder?

[Tito]

There are some good pics in the middle of this page:
http://www.babcox.com/editorial/bf/bf60133.htm

Basically, there are three pistons in a quick takeup master cylinder -- a large one at the rear of the MC and two smaller ones. Like Apogee says, these are a result of CAFE standards and low-drag calipers, where the caliper pistons actually retract into the calipers a bit on brake release. The QTU M/C applys a high volume of fluid initially using the rearmost piston to push fluid past the seals on the other two pistons. Then the smaller pistons take over and things work normally...