$300.00 and a lot of cursing, rear brakes still were not working up to par. WHY? I spent 2 solid weekends on this !@#$%*problem and I finally solved it. The problem is the distribution block, potentiometer or proportioning valve, it has several names. However purchasing a new one will not solve the problem. On most GTA's the front brakes do 95% of the stopping, my research on this topic indicates that the engineering of the front brakes is more than adequate to stop a vehicle except not within the limits we are use to now a days.
THE FIX!!! Buy 1qt brake fluid, a couple of good cotton rags, and a 7/8" wrench. Wrap the rags below the front of the (PV=prop valve) wrap it tight, so you don't get any on the paint. Take a small shot glass and take all of the fluid out of the front reservoir, with a rag soak up the rest. Now on the front of the PV there is a brick red flat nut/cap which looks like a big bleeder. Take it out, CAREFUL there is a strong spring inside, so keep pressure on it with your other hand. Now take it out, remove spring and of course fluid will come out, no biggy. You will now see this little tube on the inside, get a pair of needle nose pliers and remove this tube/piston.Take the spring and tube, place it on the bench and spit on it and curse it. Reassemble:A little tricky, you need to seal the hole in the middle of this nut. I tapped it with a #8/32 tap put a phillips head screw with a tiny gasket under it and it was sealed. Don't try epoxy or silicone, brake fluid is semi acidic. Or go to your local auto store and try to find a manifold plug that will work. Put the nut or plug back in, DO NOT touch the brake pedal, have someone sit in the car while you get your wrench on the nut/plug. Tell them to SLOWLY press the pedal down while you loosen the nut until air/fluid comes out. Do this 3 times SLOWLY and it will be bled properly. Take it for a ride, YOU OWE ME A STIFF DRINK!