1994 Toyota Corolla Low brake pedal

Brakes problem
1994 Toyota Corolla V4 Two Wheel Drive Automatic 124000 miles

I have owned this car for 9 years and about a month ago the break peddle started going almost all the way to the floor. I checked the break pads, they were still at about 80% of the original thickness. Checked the rear pads and they are still fine. Blead the lines and had the same low peddle height. Replaced the master cylinder (bench blead it first) and reblead the entire system. It seemed to make a little differance. Tested the power break booster, it failed so I replaced it (adjusted the break peddle to the max height and the push rod to the no clearance spec recommended) and reblead the system again. It made no differance. Replace the wheel cylinders, readjusted the rear breaks, and reblead the system again. No differance. So with the only thing not replaced being the break calipers, I replaced them also and again reblead the system. All this has made no differance. I thought with all the things I replaced that some air might have gotten into the master cyclinder so I bench blead the master cylinder again and reblead the system again. I've done this stuff to plenty of other vehicles in the past and have allways had possitive results. At this point I don't know what to do. The paddle goes down to less than a 1/4" from the floor. Am I missing anything?