I drained the master and I was mistaken about there being a filter so I removed that master. Bench bled the mew master and thee was a flow of brake fluid from both ports when I was was done. Got the new master in the GP and gravity bled the rears. The pedal felt like a normal brake pedal and after pumping the brakes a few times I gravity bled the rears again, which with the old master would not gravity bleed after pumping the brake pedal. So it looks like that problem is cured. I have to get eh back weels back on and get the back end on the ground, bleed the front brakes and go for a test ride this weekend... 🙂
I'll take the old master apart to see what the problem was when I have time