¿Clutch pump problem or other source of air in the hydraulic circuit?

Hello: I have a Grandis 2.0 Di-D manufactured in 2005, with manual transmission.

The clutch pump of the Grandis was replaced after 150,000 km because the clutch pedal didn't lift up and it didn't engage in 5 or 6 gear at 800m altitude, with low atmospheric pressure. The pedal only returned to the fully up position at the sea level (more atmospheric pressure) in first or second gear.

The problem has reappeared after 15,000km. Is it possible that a clutch pump only lasts 15,000km?
Is there any other possible source of air in the hydraulic circuit?

Many thanks