1996 GMC Sonoma clutch

One cold morning (15F) the Sonoma started shifting normally. After a start from the first stop sign the clutch action got softer until a few shifts later the clutch had no resistance and I couldn't get the transmission out of gear. The clutch reservoir level was down an inch, but still had adequate fluid. I topped it off, pumped the clutch enough to barely get 'er in gear, and limp the truck home. The clutch is original to the truck. I see no obvious fluid leakage.