There is a small amount of free play in a hydraulic clutch system. I believe there is a compensating port in the bore of the master cylinder that allows fluid to equalize between the cylinder bore and the reservoir when the clutch pedal is released. A small orifice (the compensating port) is drilled and machined at a precise location in the bore so the lip of the seal is not damaged when passing by the orifice. The compensating port allows for thermal expansion / contraction of the hydraulic fluid when the clutch pedal is not being used. Hydraulic pressure builds after the piston seal passes by the compensating port, hence a little free play in the pedal.
- John