Lose hydraulic clutch control in cold weather

My Ford Ranger clutch hydraulic system loses fluid at low temperatures. I can find no leaks so I figured it could only be slave cylinder and bought a new one, but never put it in because in summer everything is OK, never any loss of clutch funtion. Now in the winter every cold night there is no clutch in the morning. I can pump it up, pour in more fluid, and everything is OK until the next cold night. There is no leaking fluid I can find and no loss of clutch friction once I pump it up enough to use it. That is, I don’t think fluid is leaking from the slave cylinder because it doesn’t seem to splash and contaminate the clutch plate.

Then if it gets cold, down toward freezing or below, the clutch goes out again. What is going on here?

Look under the dash where the clutch pedal rod passes thru the firewall, and peel the carpet down to see if that’s where the hydraulic fluid is leaking.