You won't see anything with a generic OBD scanner. You need a MB tool for that.
Don't touch the Master Cylinder, it's unlikely that's the cause. I would suspect the ABS sensors at the wheels, since that is what they played with.
You really need someone to read the fault codes in the BAS and ABS modules. If you skip this step, you will spend more time/money on trouble shooting

The brake switch may be your fault, except that the lights came on after you had your brakes/suspension serviced. Why wouldn't you go back to the guys that serviced it and get it scanned at their expense ?
Where are you located ?

Another cause may be Air in the Hydraulic ABS pump. If the guys that serviced your car, disconnected the brake lines and did not perform the proper bleeding procedure, than that would cause the Pump to deliver insufficient pressure on the rear lines