my 96 is vacuum operated not hydroboost.

hydroboost means that the power steering system gives the added power instead of the vacuum pump

 

my guesss would be master cylinder, i had that same problem on a chevy pickup truck once, i would bleed the brakes and they would be fine for a little bit then the pedal would just go to the floor,

 

My Duster had that problem once. It turned out to be the master cylinder.

 

I am experiencing the same problem... No leaks, new calipers up front, adjusted rear shoes, and a new master cylinder... I bleed the whole system 3 times, it is a 96'. It does not have power assist off the power steering pump, it is all run off of vacuum. And yes I did bench bleed the master before installed, The weird thing was that My snap on scanner would not pick up the abs unit to re calibrate it... I am lost now!

One more thing to throw in there is that i was amazed because the front ran off the abs and the rear only had a separate Isolator valve...

 

99 percent of the time this is due to a cut lip in the master cylinder.. I have even seen this on a new master cylinder that was supposed to be tested.... I would replace the master cylinder first if I had this complaint confirmed on any vehicle I had in my shop.... just my two cents Wayne . One more thing. you won't see this as an external leak most of the time it is an internal problem in the master cylinder

 

You know how vehicles with the power assist system go almost to the floor? This is acting like that... The truck stops great, you can feel the abs working like it should... I just thought it was weird they went to the floor like that on a vacuum boost system...

The only reason i did the brakes is cause... You would hit them and it would yank you into the other lane cause the passenger side caliper was froze up... so i replaced both, then they went to the floor so i bought a new master cylinder...

 

I'd say that it's the master cylinder too. Sometimes the new ones are bad. While you had it apart did you install the SS braided hoses? They'll give you a firmer feel than the rubber ones. The cunifer lines are a nice upgrade over the steel lines too, takes the rust and corrosion issue out of the picture.