Clutch pedal fails in cold weather

[spoon1956]

Living in Colorado we get extreme weather variations. On mornings the are very cold < than 15 degrees, my clutch pedal does not return to its normal position without me pulling on it with my toe. I have had the cylinder rebuilt, the lines flushed (3 times) and still I have this problem,

As the temperature warms, the pedal returns to its normal position and with normal engagement (pressure). Is there water in the line freezing and keep the fluids from moving?