Brake pedal stops normally then slowly fades to floor. Master cylinder? Booster?

[Rotmilky]

An update on my ongoing brake fight. I've still got the exact same problem. Here's what I've done to date:

1) Replaced the master cylinder. Twice. That's right. I'm on my 3rd master cylinder. All do the exact same thing. So, unless I'm the unluckiest person on the planet when it comes to Master Cylinders (and they're all broken out of the box), then it's not the MC.

2) New brake booster didn't fix it. Yea, I didn't think it would, but I was grasping at straws.

3) Replaced all brake lines with braided steel. Same issue. The lines aren't pinched nor are they 'ballooning'. Not the brake lines.

4) There are no leaks. I've checked all the connections repeatedly. Thinking I might not be looking at the right spot, I sat in the idling car and pumped the brakes for about 15 minutes. That's about 45 swings of the pedal to the floor. When bleeding the brakes, 3-4 pedal pumps will take the reservoir from max to close to min. After 45 pumps, it was visually in the same spot (I put a pencil mark there). So, no leaking.

5) At this point, I've now purged gallons of brake fluid through the system with a motiva bleeder. And I'm not kidding. I think I'm up to 3-4 gallons. I'm using Pentosin Super DOT 4 because it's the cheapest DOT 4 fluid at the auto parts store. When I'm going through gallons of fluid, I can't afford Porsche OEM. In any event, I can't imagine there are any bubbles left in the line. Even if there were, the brakes aren't spongy. They're firm...but slowly lower to the floor. So, pretty sure it's not purge issues.

So, I'm moving on and am going to start replacing parts one-by-one downstream from the MC. The next two up are the proportioning valve (pressure regulator 986 355 305 00 according to the parts catalog) and then the ABS.

If anyone has ideas or suggestions, certainly send them my way.