With all of the new hardware, it appears that you've covered all your bases at the wheels. This leaves the m/c as the likely culprit.

When the problem occurs, crack the line at the m/c to relieve any pressure to the rear brake system. On the W126, the forward line on the m/c goes to the rear brakes. The W123 is likely the same, but I'd appreciate confirmation from someone who's got one.

If the problem goes away (before you hit the brakes again).........the m/c is the culprit.