Still having trouble bleeding brakes!

[My72Cheyenne]

Ok. Here is the deal.
I've been trying to bleed the brakes on my 72 3/4 ton 4x4 (disc front, drum rear). I bled the master cylinder and prop valve as "one" and things worked fine. I go the right rear to start bleeding the wheel cylinders and I get only one good shot of fluid before the piston in the proportioning valve shifts and doesn't allow fluid to the back brakes. I then have to re-bleed the master cylinder and prop valve to get the piston back into position and get fluid flowing again. Has anyone else had this trouble? I'm pressing the pedal very slowly and seem to be following traditional methods for typical brake bleeds, but somehow that prop valve is messing me up. HELP!!!
Tom,
Bend, OR