Mowog1800

Hi all. My 97 MGF has developed a strange brake issue that I'm having trouble diagnosing.
When I push the brake pedal, it goes long, about half way to the floor. The brakes work but don't instil confidence. If I pump the pedal, the pedal stays high, where it should be, and feels rock solid, no sponginess at all. I start driving again and next brake application, the pedal is low again until pumped and again it comes up and feels solid. Brake system is factory standard with new rotors and pads all round a thousand Kms old.
Here's what I've done. Bled whole system twice or three times, replaced brake master cylinder, replaced servo, removed, inspected and refitted all calipers, no seized pistons or seized pads, lubed all caliper slides. Clamping off all flexible pipes one at a time didn't change the symptom (car was in the air not driven).
So the system appears to be in good order yet still this long pedal at first press.
I am unfamiliar with what kind of problems you get with ABS modulators. I've worked on my own cars for 40 years but this is the first with ABS. There's no obvious way to bleed the modulator.

If anyone else has experienced this issue and can offer any insight, it would be greatly appreciated.

Thanks in advance. Scotty.