In order for the brakes to work effectively and quietly, Alfa designed a stepped (recessed) area on the portion of each caliper piston which presses on the back of its pad.
This will become more clear upon examination of the pistons (you cannot see this with the pads in place)—or by taking a look at the diagram below, which shows a caliper that has been split open for a better view.
The idea is that the stepped-down section of each piston should “face” directly into the rotor’s travel. For the geometrically-inclined: the step would follow an imaginary radial line from the center of the disc.
 When you rebuild or replace a caliper, cut out and use the templates (PDF 101kb) to check that the caliper pistons are rotated to approximately the correct position.
If you’re rebuilding the caliper yourself, install the pistons at the correct angle. If you’re working with rebuilt calipers, be very careful, when turning the pistons, not to damage the pistons or their dust boots. For this operation, I have successfully used various pieces of hardwood. Others have used a battery terminal expander (probably on the rear pistons). A scrap of leather can prevent a metal tool from doing damage. Whatever you use, don’t scratch up the pistons with pliers!
- Mark Lee
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