Suburban Brake Blues

1995 GMC Suburban with ABS. I replaced brake pads, shoes, rotors, calipers, wheel cylinders, master cylinder and brake booster. While bleeding the brakes I learned that no fluid would come out the rear. Only the front. Traced it back to the ECHU (valve near master cylinder). Is it definitely the valve or could it need some special bleeding procedure?

Try asking the ? on the Chevrolet Suburban side . Dave