It has been recorded and addressed as far back as 2009 that when replacing certain Clutch Servo's, leakages may occur. Fluid from the hydraulic operating valve may then escape from a ventilation bore in the clutch servo.
This situation generally occurs where there is poor comination of tolerances between the flywheel, clutch disc and pressure plate. When the clutch pedal is activated, the clutch servo push-rod is operated outside of its specified stroke range and fluid leaks past the valve seal.
It has been documented that this issue may arise in MAN vehicle models TGA, TGS, TGX, Citiliner, Trendliner, Lion's Coach, Lion's Regio and Omnibus.
Clutch Servo Part Number 907 051 412 0 should be replaced with
970 051 466 0
The unit has a longer push-rod that does not operate outside of the permitted stroke range even in the above situation.
See MAN Service pack 274903 available from MAN for more information.
NOTE: We would also advise that activating the hydraulics by way of a test can destroy any clutch servo if the push-rod is not limited to the stroke range.