Lbg500

Need some help diagnosing what is wrong with the power steering on the G. When backing out of a parking space and turning the steering full lock to the right, i lose power steering. i have had the vehicle at the mb dealer three times now to fix this and now they are saying that this is part of the design of the steering system... needless to say it sounds like they are just done replacing things that are not solving the problem. just got it back and abs light came on so back to the dealer in a week or so. when i take it back i want to be prepared with more info so i can have this issue fixed before my cpo warranty runs out next year. any advice on this would be helpful. thanks in advance.