[mpumas]

There are pressure and no pressure conditions in the brake system. I don't think there is ever negative pressure (vacum). The piston retracts due to a sealing O ring on the piston that does not normally move when the piston is moved out when the brakes are applied. When the brakes are released, this O ring pulls the piston back enough so the pad doesn't touch the rotor. Releasing the brakes doesn't pull the piston all the way back into the caliper assembly which negative pressure in the system would. That is the self adjusting feature of the front disc brakes.