Unusual brake problems

[SGFMORover]

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?

[SGFMORover]

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???

[SGFMORover]

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.