scotch wrote: I guess they're all wrong.
Not going to debate whether the "seals and boots" retract the pistons. For the purpose of clarity - The spring in the M/C returns the M/C piston - thereby pulling the caliper pistons away from the disc. when the lever application pressure is nil. Fluid Dynamics !
The ports are swapped on your second diagram too. On a modern motorcycle, the smaller port, the equalizing port/compensating port/expansion port.... etc., is in front of the piston.
It's not really fluid dynamics like it would be with a hydraulic ram circuit. Unlike a Ram circuit, the cone shaped main seal on a single cylinder master cylinder piston allows pressure to build when it's moved in one direction. When the piston retracts into it's bore, fluid can move around the seal - i.e. from behind, and this is what allows fluid to be replenished in the high pressure side of the circuit (along with the equalizing port).
