if you look on Page BR7, of the WS manual, is shows the ABS brake cct, and sure enough, it is 3 way feed, front wheels are separate, but rear wheels do indeed have a single feed, with a "T" on the back axle. There are 5 pipes, but the other 2, are the dual cct from the master cylinder, separating the front and rear systems, rather than the more conventional opposite corners, as shown on page BR37
What I can't work out though, is where the flexible pipe you are clamping off is, e.g., before or after the "T" piece. I think IIRC it is before, passing from the body the the rear axle in the middle, so when you clamp it, it is actually stopping the braking to both rear wheels.
My recommendations are:-
Make sure that the flexible pipe is not getting pinched somewhere when the rear axle is back in it's normal ride position.
Also check the handbrake cable is not snagging on something, so when the wheels are back on the ground, it is pulling the cable up.
Have you looked at the load sensing valve?, although I really can't see how that could keep pressure there, since it reduces the pressure.
Personally, I would do a part by part check to make sure the rear brakes are assembled correctly, and that you have the 2 retainers fitted to hold the shoes against the back plate. I would then get someone to gently press the brake pedal (Not to far, you don't want to pop the pistons out), and watch how the shoes move out on both sides, and hopefully returns via the springs, as it sounds like the springs may not be pulling them back properly.
After that, since the ABS pump does have a separate return valve for the front brakes to the rear, it could well be that, but again, other than replacement, I have no idea how you could test it.
