try using a 200sx 1980-84 master swap in your pushrod and the bias will be right for disks on the back. the stock m/c is biased for drums and will not provide enoug fluid to operate the rear disks correctly. I have done the swap severall times on 510(1600)s and ran into the same problem, once I figured out the fix I had no problems out breaking corvettes and supras (I was going 40-50mph slower admittedly)