Pedal goes to the floor... And i am stumped

I'm not by any means a brake master but have done a lot of brake jobs in my time. One time I did a master cylinder replacement on a Chrysler lebaron ,I think it was around a 98'
model. I bench bled the master and all four wheels and had and good pedal until starting then no pedal at all. Pulled the master bled it again and all 4 wheels with same results
and fooled with it for about 3 hours checking the lines,hoses and wheel cylinders and calipers. Finally had someone fire it up and pump the brakes while I bled and problem solved.