No rear brakes ABS BAS ASR indicators on 1998 C280
 

I will be as concise as possible in outlining the history behind my question.

The vehicle was out of service for about 2 months, during that time the rear wheel bearings, and brake pads ere replaced and the front wheel bearings, tie rod assembly, lower ball joints and brake pads were replaced. All sensors were checked for damage and all hubs ere cleaned in the pick up area. Wiring connections ere double checked for security.

Upon putting the vehicle back into service all systems indicated clear via the instrument panel. The brake pad felt soft with the engine off and also when the engine was started. At the time I attributed this to the initial repositiong of the brake pistons/pads to the rotor.

While exiting the gravel drive the ASR light blinked as normal when one of the rear wheels spun. On the next brake application after the ASR light blinking the BAS, ASR and ABS indicators came on and have been on since. The ASR indicator does not come on when the wheels spin and there are no rear brakes. (Both rear wheels spin freely in grave on on grass while the brakes are held on)

The Bentley manual suggest that low battery voltage could cause triggering of the indicator lights. The engine had cranked slowyl when started so the negative battery lead was disconnected and the batter charged at 15 amps, it took about 2 hours to reach full charge. The cable was reconnected and the vehicle started and driven but the indicators stayed on and the rear brakes were not working.

Under traditional pre-ABS diagnosis I would replace the master cylinder based on the symptons of no rear brakes, spongy pedal that would "pump up" some but still no activate the rear brakes and no fluid was lost or lines disconnected that would have introduced air into the system. I am not knowledgeable enough of the actual operation of the ABS system to know if an error or fault in it would produce this problem. If I had the appropriate fittings I would put a pressure gauge on the master cylinder discharge ports to check its function.

My questions are: Is the master cylinder a prime suspect here and if it is bypassing in the rear brakes will the installation of a new cylinder restore functioning of the BAS ASR and ABS systems and turn off th efault indicators? Can an OBDII reader pick up the ABS faults or is that in the domain of the factory diagnostic tool.

T W Hudson
hpshooter@juno.com