stock proportioning valve piston stuck?

[UGABRONCO]

I have a 76 with a stock proportioning valve and the brake warning light sensor. I know that there is a piston on the PV that slides to one side when there is an imbalance to prevent the complete lost of fluid. Is it possible for that piston stick if it sits in that postion for a long time. Basically the bronco was driven without rear brakes and parked for about a year, and I have been trying to bleed the rear brakes and I cannot get any fluid tom come out of the bleeder screw. Sorry for the long post, but I am so close to finishing the rear disk setup and driving the bronco again.

 

[j.r.nice]

On the front of the prop valve there is a little "pin" that sticks out horizontally from the body's end. The pin needs to stay out to bleed.