RTFM.
Page 26 of the Technical Manual has procedure for loosening sticky pistons:
http://www.ciclocoimbroes.com/docs/A...al_tecnico.pdf Here's an excerpt.
Before completely disassembling your caliper,
it’s worth trying to loosen the sticky piston. Try
the following: Clamp bicycle in bicycle work
stand. Spin affected wheel. lightly squeeze
brake lever and watch brake pads when lever
is released. determine which side of the caliper
has a slow returning brake piston. Remove
caliper from bicycle. If you have a mounting
bracket, it is recommended to remove that too
or just remove the caliper leaving the bracket on
the fork or frame. Remove e-clip from guide pin
groove on top of the caliper. Using a 2.5 mm hex
wrench remove the guide pin from the caliper.
Pull and remove both brake pads and h-spring.
Using a 10 mm box wrench, press working
piston into caliper body. Squeeze brake lever
slowly to move sticky piston inward. Press
the piston back into the caliper again. Repeat
these steps to correct caliper piston inner o-ring
position. Both pistons should now be moving
freely. Re-install spring pad clip, h-spring, and
pads into caliper. Re-install caliper onto bicycle.
Spin wheel, check function. If there is no
improvement, continue with caliper service.