74 Dart w/ 340 6 pack

I just want to beat my head against the wall.:banghead::banghead::banghead:
This weekend I put in the new master cylinder, brake booster, proportional valve and then bleed the brakes again and again and again to no avail. I would get a good hard peddle no bubbles in the front and it would stay that way for a while 10-20 minutes then soft again with air coming out……….Checked all connections no leaks, I did all new brake lines when I did the rebuild, front, rear, to proportional valve so I don’t think it’s the lines. Ideas??????
Ended up having to stop playing but I will be at it again next weekend………:wack:

Could be a bad master. I got one and spent hours trying to bleed the brakes. I went and got a replacement and worked right away.