The dealer who supplied the car is saying the dealer inspecting the car cannot provide them with a cause for the fault.
In that case then ( like I have said before ) the car needs to be returned to the ORIGINAL dealer, in order for THEM to do their own inspection and formulate their own opinions of the cause of events that caused this total brake failure.
Come on both dealers, it pretty obvious to me at least what has happened here, its an assembly drop off at the factory, pure and simple.
For reasons we will never know why, that flexi brake hose was NOT correctly located and then held in place by the retaining clip in that bracket, that is fixed to the body shell of the car.
The bracket likely had some type of defect, that prevented the retaining clip from being installed at the correct time in the assembly process ?.
Look at the picture, where is that all important retaining clip ?.
It never got installed - clearly.
So, the metal brake line gets fitted to the flexi ( regardless ) and is then tightened to the correct spec's.
But because the flexi is not secured to the holding bracket, the tightening affect of the metal brake line, has then in turn twisted both the metal brake line ( seen in the picture ) and the flexi as one item, towards the direction of the road wheel.
The necessary clearance between the brake flexi and the rim of the road wheel has now been greatly reduced and this foul condition has compromised the external wall of the rubber flexi.
Each time the pipe comes into contact with the wheel rim the brake line degrades even more.
Reaching the point where brake fluid then starts to leak each time the footbrake is depressed, slightly more fluid is slowly lost.
Then suddenly the wall of the pipe completely fails to the point where all foot brake pressure is lost instantly as all of the brake fluid has been ejected from the system.
I think it would take a vehicle inspector less than 10 minutes to come to a similar conclusion myself.
THE CLIP - THE CLIP is was never installed in the factory.
If you DO decide to have it independently checked and supported with an engineers report, I would suggest that you inform the original dealer ( in writing ) that is what you intend to do to support any future legal actions
As the dealer who is holding the car right now, is clearly distancing themselves from the situation and unwilling to get involved.
It is a "pass the parcel" situation unfortunately.
The dealer who supplied the car wants the holding dealer to pick up the baton, but he is too smart to fall for that one.
Here you are, stuck in the middle !.