C3 no brakes suddenly then ok !

I have an 82 that I recently did a brake service on, new O rings on 4 calibers, new master cylinder, bleeding..brakes were good but not perfect, pedal still goes a long way in, drove for 3 days then took it to another service place and additional front caliber bleeding, pedal got better, drove it for 20 minutes and was perfect, then at a stop light put the car on N, when I drove it the pedal went all the way to the floor and no brakes! Luckily I managed to get her home safely, it was scary I tell you, but the next day the brake was fine ! Drove it carefully to the first place of service to figure out what went wrong and everything was ok, oil was full no leak, been driving it carefully for 2 days and still fine, what do you guys think was the problem? How can I make sure it won’t happen again?