72 K20 brake help

[kryszkuk]

I've scene it posted before but can't find it now so sorry for the repeat question in advance.

I can't get my 72 k20 to stop. Changed the master cylinder, wheel cylinders, all rear brake springs and shoes, rear hose and brake fluid. Bench bled the MC, reset the proportioning valve, adjusted the shoes tight to the drums, and bled the brake lines till I got good clean fluid. My peddle goes down 1/2 to 3/4 of the way then good and hard. At a speed of maybe 2mph it stops on a dime but anything over that its like I have grease in my brakes, it barely stops. The only other thing is at idle and in park if I step on the brake peddle the RPM's go up a tad then go back to normal. Is it the proportioning valve or brake booster? Any advice is appreciated