yes, i mean the rod that comes out of the booster and engages the master cylinder, sounds like it might be to short, there is a tool for adjusting it to the right length, but i just turn it out until it wont let the MC go all the way on without pushing the rod, then i turn it in to where it will just go on flush and just touch the rod. that should get you really close. Had the same issue with one of my old trucks and that fixed it completly.
I dont remember the Prop valve having a bleed nipple, i thought they just pushed the fluid through and bleed at the brakes, have to force the valve in the prop valve one way or the other to get the fluid to bleed to the correct set of brakes, (front or back). but i may be wrong, been a long time since i messed with one of them, just sounds to me like the plunger rod for the MC.