Unusual brake problems

So, I have a braking problem on my 95 d1 that has me totally stumped. Pedal was travelling too far and booster was hissing like mad so I replaced the power brake booster< hissing got better and pedal drops correctly after starting - but pedal still travels just shy of the floor before gradually stopping. No leaks, bled sequentially over and over, stainless braded lines and all 4 calipers are working evenly. figured it had to be master cylinder so I picked up a new (not rebuilt) one from local parts store, bled again.... HARD as a rock and barely travels at all. bad part? order different brand try again - same - hard as a rock, takes both feet to budge pedal and slowly stop truck. switch back to original master cylinder, bleed, firms up until truck is turned on then drops to the floor. so I have one master that is too soft and 2 that are too hard?! pulled the fuse for the ABS - no change. put back in old booster, no change except gradual leak of pressure with truck off so switched it back out. anyone know a way to check the pressure reducing valve? any one know how to confirm what might be wrong?

Have now swapped pressure reducing valve and abs modulator with ones from salvage yard - no change. trying a 3rd master cylinder today - though it too looks different than the original lucas model. still open to any ideas???

and the winner is - master cylinder. For anyone who ever runs across this there is a new/reman master cylinder being sold aftermarket under atleast 4 different names between local parts stores and online retailers - IT WON'T WORK..... if it doesn't look like the old Lucas style it doesn't allow the power brake booster to function/does not allow enough pressure to be passed on. It looks like a heavier duty version of the exact same design but after 5 of the same thing from 3 different places under 5 brands I picked up a $20 used lucas master cylinder from a salvage yard and have perfect brakes for the first time in years.