Brake pedal goes all the way to the floor when the engine is started

We have been trying to fix the brakes for a week and they pump up, but when we start the car the pedal goes all the way to the floor. Brakes have been bled, front hoses replaced, new pads and calipers.

Before I got the car someone installed front disk brakes. They also installed a brake booster and master cylinder. It appears they used GM parts for this. In a mixed brake system shouldn't the rear (closest to the booster) reservoir (for the disks) be larger then the front reservoir (for the drums)? Can anyone recommend the correct part numbers for the master cylinder, booster even calipers and rotors? Part numbers would be great. I have got a feeling the buy cobbled together this brake conversion as cheap as possible. Thanks in advance fr your time and your help!