I don't know the percentage of clutch pedal pressure that comes from the pushing of hydraulic fluid in master & slave versus the power of the return spring for the pedal. So, I'm not sure if the hydraulics could not be working at all, but to be fooled by the heavy return spring. As Tom said, it's rare, but occasionally you get a bad unit from the manufacturer or rebuilder. If you had a fitting to close off the tubing that goes to the slave, that would tell you if the master cylinder is good.