More 964 ABS AWD Issues - Suggestions?

[1990C4S]

I have persistent warning lights that I can't get rid of. I get the red ! warning, and the red brake light, with the occasional amber anti-lock light.

What I've done:

Checking the wheel sensors I found three sensors at 1,000 ohms, one at 1,700 ohms. The high resistance sensor was replaced. Scantool did not report a fault.

I did get a 'longitudinal sensor' fault from Scantool. I cleaned the sensor up, that error is gone (I think that repair also cleared the amber anti-lock problem).

I have bled all the brakes, I have not bled the ABS or the AWD yet, they on the to-do list.

Observations/Status

I get the red ! warning, and the red brake light almost exactly 20 seconds after I start the car, whether I am moving or not.

Scantool does not show any reported faults, but my Scantool will NOT connect to the ABS when the car is running. So, a real fault, but no stored code.

I tried engaging AWD from the dash switch, the rear end sounded 'clunky' in a 'this might do some damage' kind of way, so I stopped and turned it off.

Questions:

Anyone know if the Scantool should connect to ABS when the car is running? I can connect to Motronic, and do adaptation when it's running, but not ABS.

Anyone know the sequence of testing in the ABS logic? What is ABS looking at 20 seconds after starting? It can't be wheel speed sensors, because I am parked when I get the fault, they get checked above 5MPH. Maybe it's ABS system pressure? But I don't think the pump is running when I get the fault...

Is bleeding the ABS & AWD the next logical step?

This car is getting frustrating...

[1990C4S]

After a lot of digging, it appears that there was a LOT of air in my ABS brake system (accumulator). It took a lot of bleeding, then a lot more bleeding, but I am now able to drive a mile or two without an ABS fault.

Longer test pending.

Time will tell if I have an air ingress issue, or I just had a lot of air in a system that sat around for years.

I can confirm that an ABS pump that runs almost silently is compressing air, and a pump that is actually building system pressure can be heard from the driver's seat...

And my AWD problem was simply the result of tire scrub turning with diff lock engaged. In a straight line it is fine.