Jimmy '88 with bad rear brakes

[dmforsell]

Hey guys!

I have a GMC Jimmy from '88 with a rear brake problem that I seem to be unable to fix and would be very grateful if anyone could help me with my trial-and-error.

The problem is that when I apply the brakes I get weak pressure on my rear brakes and if I simulate an emergency braking situation on gravel I can see that the rear brakes don't lock or if they even work. My front brakes work perfectly.

This is what I've done so far:
* Renovated the Master Brake Cylinder
* Fitted a brand new Brake Booster
* Changed all the Brake Lines and Brake Hoses
* Fitted brand new Rear Pads and Wheel Cylinders.
* Bleeded the system several times starting with rear brakes bleeding atleast 6 times with front brakes valves open, then shutting and bleeding front brakes valves atleast 6 times until I'm satisfied with the bleeding.

But still I never seem to get the rear brakes to work.

Now, what else could I try? Is there something I'm overseeing?

Could a defect Proportioning Valve cause this? Or what else?

Thanks for helping!

 

[p8ntball]

I was gonna say bad Proportioning Valve

 

[dmforsell]

I was gonna say bad Proportioning Valve

Thanks!

Is there any way to test the Proportioning Valve before I replace it? I've spent so much money on new parts in the last couple of weeks that I'm not very keen on replacing another one just to find out that's not the problem.

When I've been searching around for information about this I've stumbled upon some people saying the Master Cylinder needs to be benchbled, is this a very vital step that could cause my symptoms? I mean I've bled it several times both rear and front about 20-25 times now, could it really still be air in the Master Cylinder? I've bled it the usual way by having a friend gently pressing the brake pedal while I'm receiving the fluid.