Reply to "Sinking Clutch Pedal"

I've had this myself, though not on a Pantera.
The cause of the trouble was the spring inside the master cylinder that is meant to return the piston to the start position. The spring had broken, so sometimes the piston returned and the clutch would work. Sometimes it stuck 'down' so that when I pushed the pedal it wasn't doing anything.
Dirt inside the master cylinder could also cause this, either by making the piston stick or by getting under the edge of the piston seal and letting the fluid pass it by.
Hope this helps.