This is just a guess. But with some cars if you allow air to get into the ABS pump you will have to bleed the brakes with the use of a Bosh Hammer or a PST II, these are the special tools used on Porsche brake system if the ABS pump gets air into it. The special tools activate a soleniod that I think activate the ABS pump to bleed it. Maybe you have to that on a MB too it the ABS pump gets air in it.