1969 Cadillac coupe deville brake question
 

Hey everyone , I’m new to the forum , hoping someone out there can share some
Info with me . I have a 1969 Cadillac coupe deville I’ve been working on for a while , Disk brakes in the front and drums in the back , I recent replaced the whole brake system , brake lines master cylinder , brake booster - calipers - every thing , I bled all of the brakes and don’t seem to be getting great pressure at the rear , but besides that , once I get everything bled the pedal feels good , good amount of pressure and it returns back up strongly . As soon as I start the car and push the brakes it goes to the floor , for the most part all of the pressure is lost . Why would this happen once starting the caddy ? I know the brake booster has a vac*** line to it from the motor . If anyone can help me any info would be greatly appreciated , I really want to get this thing up and cruising ! Thank you !