Are you referring to the caliper pistons? You will have to remove the caliper (leave the brake hose attached) to inspect these. After ypu get the caliper free from the rotor , remove the brake pads , push one piston in and this should push the opposite piston out . If it doesn't presss the brake pedal a couple of times to extend the piston. Stop when you see a clean surface on the piston.Use Brakekleen to remove all dust,etc. from this piston.You can use a shop towel to wipe off dirt. When you are satisfied with this , push that piston back into it's bore til it is bottomed out . Do the same to the opposite piston.
I do not use ANY petroleum base products on brake parts. It is a good idea to look at the pads to see if they need replacing. Make sure the friction material of the pad is facing the rotor. When you are reinstalling these parts, make sure you are doing this correctly and tighten all fasteners. Try your brakes on the bike stand and not on the road. You do not want to hear the sound of parts falling off your bike !