First off I think you have a bad master cyl, bubbles like you describe should not be happening. You can upgrade to a 1" bore master cyl from a 89+ V6 truck. Also the LSPV (the thing you called BPV?) is a Load Sensing Proportioning Valve, it controls the amount of fluid getting to the rear brakes and increases the flow when you have a heavy load in the bed because it moves the rod attached to the axle. If you have a lift on the truck you need to raise the end of the rod that attaches to the axle a like amount. You can actually strap the rod full up to the frame and it will be fine, you will have maximum rear braking all the time so make sure your rears don't lock up before the fronts or you could swap ends when panic braking. Definitely change the master cyl and then bleed the crap out of the whole system, shoudl work fine.