the brake master cylinder seals are the most likely cause .
the X5 has 2 brake circuits , one for the front brakes and one for the rear brakes .
a forum member had this same problem last year and had difficulty finding a mechanic with enough knowledge to find the problem .

the car required a new master cylinder , simple as that .cost a few hundred $ .
you need to have the entire system checked properly . checking all the calipers work properly .
you should not drive the car at all until it is repaired .