Update to brake issues

What is your question?

Here is the latest update, have Les Schwab replace master cyl., they blead out the system. The pressure was fine for a day or so then back to slowly loosing pressure. Took car back they went over entire break system and found nothing wrong. It was suggested that since the pressure seems to bleed away after sitting idle that it is possibly a vacuum issue where the engine is robbing the booster of pressure. So the question is where on the echo system is there a one way valve that regulates the vacuum pressure between the engine and the brake system?