If drum brakes consist of a several springs inside and outside the fluid cylinder to retract the brake shoes after braking, how disk brakes piston, that dont have any springs, can retract alone without any force pulling it back,can vacuum only pull the piston after braking??
The piston is hold in place by a rubber-gasket. The piston does not slide in this gasket, instead, they stick together.
So, when the brake is activated and the piston moves towards the disk, the gasket deforms a little to allow this movement without sliding.
When the brake is released, the gasket moves to its original shape and retracts the piston.
However, a little bit of sliding is necessary over time to compensate the wear of the pads.
I found this animation, which clearly shows how the (red) gaskets deform and retract the pistons, and how the piston slides when the pads wear out: