Have just replaced brake calipers on rear and piston kit in front calipers. Have bled brakes several times and no air but let pedal sit for a couple of seconds and then it pushes straight to the floor. Pump it up and hold it and it appears to hold pressure. Let it off then push again and it h Goes straight to the floor again. This happens when engine off and engine running. The master cylinder and booster weren’t touched during refurb.
I would start with the master cylinder. On older or high mileage vehicles the master cylinder bore will develop wear. The wear only occurs in the area of the normal piston stroke. In the process of bleeding the brakes you pushed the seals past what had been the normal stroke. It likely ruined the seals and you are either sucking air in when the pedal returns or pushing fluid past the seal on compression.