Weird W126 Brake Problem

[mramay]

Picked up a 380SEL with my son yesterday and besides the little niggling problems (e.g., wiper blades, paint, snafu window lifts), the brake pedal was almost to the floor before it was able to stop the car. The brake reservoir was pretty much empty. Added fluid and tried to bleed the brakes but couldn't seem to be able to get fluid out the back brakes.

Put in a new master cylinder, bled the brakes and got nice clean fluid out of all four corners now. Verified all four corners had great pad thickness and nothing appeared too hot, like a stuck caliper

But the brake pedal still goes too low. It's better but no where near my 560SEL pedal.

Now what???