RE: 1985 FORD F150 brakes
well, your brake fluid has to be going somewhere. it can't just disappear. i don't see how the proportioning valve could cause the problem you describe. possibly the metering valve could be bad and that will cause a low squishy brake peddle as well as low or no vacuum in your booster. when you removed the booster was there any brake fluid in the diaphram? that could be where your leak was if not in the lines, master cylinders sometimes leak out the back into the booster. check your wheel cylinders in the rear drums, just use a flat blade screwdriver and pop a side of the rubber sealoff of the ends and see if its full of fluid there should'nt be any in there. if there is that indicates seeping. its not technically a leak until it actually gets past the seals. oh, one more thing... if your truck happens to a residual check valve for the rear brakes, which if a its a 85' it should'nt, but if thats the case your check valve may be malfunctioning and will explain the low pedal and good pedal after pumping. there are a lot of possiblities to your symptoms, way too many to list, check back and let me know if this helps or if its greek I'll try to explain better.