1 Front seal fails, or pitting of cylinder wall. Fluid leaks back towards the reservoir instead of pumping out the outlet pipe. This will result in reduced or no slave piston movement. The effects may vary with a borderline seal. (rebuild or replace master cylinder)
2 Air in system- master, line, or slave cylinder. Air compresses, resulting in reduced piston movement, as above, & soft pedal. (bleed system)
3 Rear seal fails, fluid leaks out the back of the master cylinder body, possibly onto the drivers foot. (R/R master cylinder)
4 Piston doesn't return completely due to crud or rough surface. Cylinder remains partially pressurized, and slave doesn't return, leaving clutch in intermediate state. This may leak back slowly. (R/R master cylinder)
5 Blockage of inlet - shortage of fluid, reduced piston travel, mushy pedal. (R/R master cyl.)
6 Blockage of outlet -Hard pedal &/or fluid blows back past front seal. (R/R master cylinder)
Todd