When Parker Posey first appears in the car with the brakes cut she jumps some steps and heads for some taxis...When she goes in between them you can clearly see the brake lights come on and the car gives the little jerking movement as if the car is slowing down...I thought Kevin Spacey said he cut those brakes...
Just because brake lights come on doesn't mean the brakes are working. And considering how many things the car was bouncing over & into, the "jerking movement" could have been many things besides the brakes working.
It is also possible that Lex had not totally cut the brake lines, but slit them. It is probable that with her constant pumping of the brakes, she could initially had enough pressure remaining to cause the jerking motion... But eventually the fluid would all flow out the sliced hose...