Clutch Keeps Losing Tension…'04 Jetta GLI 6spd/130K Miles (Meticulously maintained/no current work done regarding below issue)

Clutch seems to have full tension in first, however, when I start climbing through gears, I lose more and more tension (When I slow down, the tension seems to come back). I keep losing very little brake fluid. The master and slave cylinders on the clutch system don't appear to have any leakage...Brakes seem soft, I know at this point there's more than likely air in the lines due to a leak somewhere...but...my question, does anyone know of any common parts that are known to go bad on the braking system that I should focus my attention to?

Any help is greatly appreciated! Thanks!