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In finding your coolant loss problem, the plastic tank has a center hole below the cap that drains any coolant that vents from the cap to the tank bottom. The VW/Audi coolant will leave a white trace when the coolant dries. If the hoses and clamps are good the cap may be venting. That would explain why a new cap does not work any better. That could mean the engine is producing too much coolant pressure that is vented above ~21PSI. The cap functions to maintain coolant system pressure at the correct level.

High coolant pressure that causes venting, especially when it occurs before any temperature rise above the 87C thermostat setting, may indicate a head gasket leak into the coolant jacket.