Two recent Helpline cases were regarding the exact same problem that occurred during an NCT testing, on two separate 2008 Mercedes C Class W204 vehicles. During the NCT test, while the vehicles were on the brake rollers performing a Brake Efficiency test, the rear metal brake lines burst. Both had burst at the at the same point during the test.
The brake lines on both vehicles were replaced and the systems were bled. But the rear brake circuit would not bleed, and only very small amounts of brake fluid were evident during the bleeding process from the rear calipers.
The Helpline is aware of this known fault , and that it is common with the W204 vehicle series. The failure to bleed is due to a faulty ABS hydraulic modulator. The ABS hydraulic modulator can be sent to a specialist for repair, or it can be replaced, without having to replace the brake control module.
This was unfortunate for one of the garages, as the car was braking normally prior to the test. The customer will need to accept the responsibility for the replacement of the ABS Modulator as this could not be avoided, as it is a requirement for the NCT brake efficiency check.
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