These calipers are not supposed to be rebuilt-- there's no authentic Brembo kit. You can't get anything from Moto Guzzi either.
I posted to this forum when I had this same problem and received similar advice. I tried to rebuild mine anyway with an after-market kit from MG Cycle. The seals (large) were not a correct fit and failed. I've rebuilt calipers in the past and it's simple with actual Brembo labeled parts. If you do decide to disassemble the caliper and see that corrosion has penetrated the piston's anodized surface, you should probably replace the caliper. To clean the pistons, I wouldn't recommend anything as abrasive as Scotchbrite which can remove the anodizing, but you might gently scour the surface with something comparable to "Bar Keepers Friend".
In conclusion, if you want to save yourself a lot of messy trouble and have reliable brakes, buy a new caliper. They're about $277. from a dealer. On a related subject, how's your rotor? If you've been riding around with a seized caliper it may have warped the rotor-- you will feel a "pulsing" sensation under light braking.
DD