BigBen, If it is not leaking in the lines where it could be obvious. It might be leaking out of the transmission, check around bellhousing and the diff, see if it leaks there. Look for a problem in the hydraulic clutch system. Sometimes the seals in the clutch master cylinder fail and won't push fluid through the line to the slave cylinder. Check if you even have fluid in the clutch fluid reservoir. Sometimes the slave cylinder on the transmission bell housing can also fail. Watch the clutch action at the clutch lever (where the slave cylinder will push the lever forward, if operating properly), while someone presses the clutch pedal. If you see no movement, one of your cylinders has failed, or no fluid is available, or the line is plugged, but that is very unlikely for a clutch line to plug up.