Here is a tricky one for the experts. I was driving along at 40mph and had to slow down. The brake pedal went 3/4 of the way down before starting to brake very weakly. There was absolutely no pressure in the system until reaching the 3/4 point in the travel. I had the impression that either the front or the rear brakes were still working a bit. As I was in a state of shock I couldn‘t do much thinking or further testing. Things returned progressively to normal after actuating the brakes 2 or 3 times.
This has happened a total of 3 times with 3 different drivers within a driving distance of 100 miles. There are no external fluid losses and the system has been recently purged. The brakes have been recently rebuilt with new seals. The car did not pull to one side when I braked.
The brakes are completely stock including the proportioning valve.
I would appreciate some help as I intend to keep being part of this helpful community.