MERCEDES BRAKE PROBLEM

I have a 2015 Mercedes-benz C200 automatic which has done only 31 000 km and is like new in all aspects. In January, I took the car in for its regular service and the dealership did a

100 000 km service because the schedule said five years must be a major service. The brake fluid was changed, among other things. Since then, the vehicle’s brakes do not function as before. When stationary at a traffic light with my foot resting on the brake pedal to prevent creeping, the pedal slowly loses pressure and drops to the floor. This requires a pump to regain braking force.

I have had the car back five times and they bled the brakes and fitted a new master cylinder but to no avail. I do not want to take chances with faulty brakes and the car is standing idle at the moment. Any ideas?

LEN BAKER

Via email

Sorry to hear about the brake issue and we are surprised the Mer‐

cedes-benz dealer could not fix the problem as it’s best placed to do so given all the diagnostic equipment and access to relevant information.

In general, when the brake pedal becomes “soft” underfoot or sinks to the floor, the following is to blame:

• Brake fluid level is too low, leading to air ingress.

• A leak somewhere in the system. This can be on a pipe, or piston seal on a calliper.

• Master cylinder is mal‐ functioning. A broken seal inside the cylinder can res‐ ult in an internal leak which prevents pressure build-up when the pedal is pressed.

• Air in the system requiring bleeding.

• ABS fault.

It sounds like the obvious checks were done by Mercedesbenz. If no leaks were found and the master cylinder changed, it only leaves air in the system or an ABS fault. Inter‐ estingly, to bleed a complex ABS system requires a diagnostic tool to cycle the valves in the ABS unit to ensure trapped air bubbles can escape. The only change during service that could have influenced the braking sys‐ tem was replacing the brake fluid. Is it possible that air is still trapped?