It appears that all the facts are not being disclosed, The master cylinder is split between front and back so if the front circuit develops a leak the rear brakes still work, and vica versa. $1300 to replace a brake line is a little far fetched. at $100 an hour plus parts they could replace all the brake lines, I would like to hear MBs side of the story, you can still downshift and use the emergency brake!
If they are willing to give you the parts. They must feel very strongly in their case. If you have always had it serviced there why would you take it to another (your) shop?
I think this was this your post in December? what did they find

Brake Light Sensor

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I have a 01 ML 55 . If the break pad sensor light comes on, is it for the front or rear break pads. ( Order my manual, take 3-6 weeks)
Tks
S
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ML55 AMG 2001
ML 320 1998