My brakes are driving me nuts! ... and not slowing me down!

[ckhd]

It's on my '72 'burb K10, power brakes. The brakes didn't work well when I bought it. The previous owner said he replaced the following: master cyl, booster, booster check valve, rear wheel cyls, all linings.

I get good vacuum to the booster, and the check valve will hold vacuum in the booster literally for days. When I bleed the system, I get a good flow of fluid at all four corners.

Now, I discovered that he used "Lucky's Really Nice Brake Shoes" for the rear, and the linings were cracked and falling apart, so I replaced those with some nice Raybestos shoes. I also made sure that the rear drums were adjusted out afterwards. I also replaced the front rotors (they needed turning and were too thin), and the front pads. At this point, I still couldn't stop, and after a little testing I realized that all my stopping was coming from the front brakes (I could lock the fronts up on a dirt road, but the rears just coasted right along). So, I changed the master cylinder. It still behaves exactly as it did with the last master cyl.

Anyone have any ideas? Is there a way to check pressures at various points, and if I can, what should the pressures be? I'm stuck at this point and I have to drive it to Flagstaff next week, and the traffic there is worse than anywhere else I've been (including Phoenix, L.A., San Diego). I'm afraid I'm going to end up smashing up my 'burb when I go.