C5 Allroad Brake pedal gets very stiff, brakes are engaging VAG shows up to 78bar.

So, My first WTF tech post. Actually, I it's my first post period!

I have a 2003 C5 Allroad (2.7l, TT auto). The other day my "Co-Star" was driving it, and after getting fuel, she noticed the car seemed to have reduced power and it got progressively worse to the point that could only barely get the car home. The brake pedal is getting very stiff. I go and look at it and realize that there is nothing wrong with the drivetrain, but it is indeed the brakes. After a quick ouch test, I discover that it is all four brakes. After a bit of VAG checking, I notice that the brake pressure was high, (about 20 bar) and for some reason, after a bit of idling and checking, I am able to move the car and back it into the driveway. It seems to work fine by this point. It was too late to do any further investigating, so I take her to work the next morning. Over the next few weeks I discover the following;

• Reading the ABS Measuring block, 5, I see pressures from about 1 up to 78 bar. (about the same as normal braking.)
• The pressure usually bleeds down gradually, about 10-15 bar a day. Sometimes it will bleed down all at once. (has only happened twice.)
• Disconnecting the vacuum line for the brake booster at both the firewall and at the booster itself resulted in no change. I didn't expect this to work since the brake pedal was also pretty stiff. The reason I tried it twice was it was easier at the firewall, but, since I didn't know if that gadget in the line was a sensor, valve or check valve, I should expend the extra effort to do it at the booster.
• Disconnecting the connector to the ABS pump also had no effect other than make the ABS indicator light up on the dash.
• On one test drive. I started it at about 5 bar. Drove is slowly trough the neighborhood for about five minutes. Pressure only went up to about 6-7 bar. (it fluctuates). I decide to take it on a faster trip on a main road, I get to the corner, hit the throttle, (Traffic and I have to dodge a street construction obstruction. I love the stage 1 tune on this!) and in about a hundred meters, I already feel it slowing. Pulling off to a side street, I'm already at 50 bar, and just barely get it to the driveway where I park it at the peak of 78 bar, having to really push the car to even make it there. All four brakes are blistering hot.
  
• The dealer has zero clues. "I talked to two of my senior technicians, and in 15 years each of experience they have never even heard of this!"
• I'm kind of afraid to post on this forum. I have no idea why.

My next step is to replace the front calipers (Both bleeders are frozen - Fracken idiot who worked on it before me!) and bleed the ABS pump.

A touch of background. The rear calipers were bled four months ago, but the system was still a touch soft. I also replaced the passenger front CV shaft a week before this, and the drivers one a month before that. I don't think any of this matters, but - just in case.

Side question, since I have to replace the front calipers, is there a compelling reason to do something other than stock? I do already have a set of new pads, and don't feel like changing wheel size now.

Well, after a long time of silence, getting the parts, and trying to make time put a brake booster (servo) and master cylinder in while taking care of my year and a half old little apprentice.

And, after finally installing them, and bleeding the whole thing out, (completely replacing the fluid) and the linkage moves freely.

It didn't fix it.

Does anyone have any ideas which direction I should go now?