I had the same issue with my b13, it did this for the last two years. I even put b15 spec v brakes in the front to help stop the car. Turns out it was bad hard lines. Both my rear brake lines had rusted right next to the fuel tank. I cut the bad lines out and spliced in new hard lines. Wouldn't you know it my brakes are rock solid and fully engage stopping the car at half pedal. I had gone over the entire system replacing everything like you did. So check your hard lines for even surface rust and replace them, it sucks but works.