1998 Chevy S-10 No (very little) Brakes

My brakes were working fine until one day when I was forced to do a full panic stop from about 35 mph.
The truck stopped perfectly, but after that the pedal basicaly goes right to the floor with only a little catch at the very end coming from the rear brakes.
The obvious problems did not manifest. The fluid level remained up. No leaks were detected anywhere. I bled the lines but basicaly found no air.
As a last gasp I gambled and changed out the master cylinder. Same situation exactly.
When the truck is not running there feels like there is some pedal, but as soon as I start it up. Right to the floor. Any ideas?