I'm no brake expert either but my 90 has had the same issue.... the brakes just didn't seem up to it, yea spongy I guess. To me I was remembering the brakes on my 72. Ya know those old iron FIXED 4 PISTON calipers w/ large pads.
Anyway enough of the old school stuff. I had the dealer check my abs against the shop SBDSytem & the abs got a clean bill of health. To get my abs to work I have to go way beyond controlled effort on the pedal...but it does work. I don't speak abs either.
Here is what I've done so far and they are better: s/s lines, DRM bias spring, rebuilt the fr calipers & rears will wait till winter, really soft street pads Brut Stop by raybestos, new hardware for the pads, ya know clips & pins & speed bleeders & Valvoline D3 syn fluid.
Now they work much better but still not what I call "stop on a dime & give nine cents change". They work but they feel like they are not. I guess they don't telegraph much back to the driver. If I unload the inside wheel a bunch in a corner & do a bit of braking I can feel the abs react to that unloaded wheel....so I'm confident it works as intended. I grew up on old stuff & pride myself in having learned brake modulation....I ain't no race car driver but I have some learned habits w/brakes pre-abs.
Oh, I did rebuild the M/C when I did the DRM spring...ya know "while we're here" rule. I even bought a new set of stock lines & may try that this winter. OH, yea, another thing if ya think ya got all the old fluid out with 3 qts of bleeding by foot...think again. My fr calipers still had some crap hiding in them. Not that that means anything, just thought I mention it. Oh, yea reason on trying the stock lines again is they have a larger I.D. than my Earl's s/s lines...more fluid movement but less pressure but ya can at least fill the rear reservior of the M/C to the fill line without it spitting fluid out under hard use....not much, very little but it spits some out...even before the DRM spring.
Sorry Ron for rambling & giving ya no exact answer. I just figured if ya had some more info ya might tumble on to something for your situation.
Tom