I don't see how you could loose everything all of a sudden. Unhooking the trans harness will take away all electronic function to include EPC solenoid, you will be able to shift it into 1,2,3 and reverse and you will have max line pressure. I don't recommend doing this unless when you hook the pressure gauge up you have a very low line pressure, and it is just to see if you can get good pressure to make sure the pump is functioning correctly and not leaking internally excessively, you want to see over 200 psi her and only for a minute or 2, I have drove them around with the harness unhooked without hurting anything but it is probably not the best thought I have ever had. Line pressure with any automatic transmission is the first place to start, this will paint the picture as to what is going on. Sounds like a major pump/pressure probelm, could be in the converter. Did you put in a new one? I don't see how a small leak could cause a probelm like the one you are having, but like you said it did work fine upon the rebuild if I'm hearing you correctly.