I do not believe that they lost braking. The picture a few posts ago shows plenty of brake action. And the Combined hydraulic reservoir is large, around five and a half gallons if memory serves, and the engine pumps can easily overcome the escaping loss and provide pressure until the reservoir runs dry. The 550 has "fuses" that shut off fluid to the nose should this occur, but I don't know about the IV. If they had been using the emergency/parking brake, they would not have had anti-skid...
It took around 25 seconds to accelerate to V1 (around 115K), and the CVR ended at 49 seconds. They accelerated to 165 Knots yet didn't fly.... Understanding this tragic accident isn't a systems question, it's a "why" question.