what causes the brakes to lose pressure when the car is started after the master cylinder and the front two brake calipers are changed and the brakes were bled all the way around and they are all receiving plenty of fluid still

There is still an abundance of air in the system - MUST properly *bench bleed* the master cylinder before installing - plus the calipers have a large volume of air in them when installed. Continue bleeding the front calipers until ALL air is purged from hydraulic system. May take *SEVERAL* attempts to do so! When engine is started - the brake *booster* is simply making the pedal easier to depress thus pedal goes down - that is normal - when ALL air has been removed - pedal travel will return to normal > provided brake components install was performed correctly..