Brakes won't bleed.
 

I'm finishing the resto on a 69 gto.
Factory disc brakes, new rotors and calipers. There are no visible leaks anywhere.

The brakes will not bleed and I'm concerned its the master cylinder or proportioning valve. I pushed 500ml through the bleeders yesterday and no improvement. At one point my partner said the pedal actually got firm, and the very next time I opened the bleeder, the pedal never "came back".
The fluid looks foamy when it comes out of the hose I attached to the bleeder, like when pouring a beer.

How low can the master cylinder fluid level go before it sucks air?

I have no idea where to go with this next.

How much of the brake system did you have apart? ex replace all lines?

I had the rear end out and replace the lines on the axle only.
the back was open for about a week, and the fron was open for about a week

Bench bled the master?

One of 2 things:
The button on the proportioning valve needs to be pressed in.
Seal on said vale leaks and allows air to be sucked in when releasing pedal.
(less likely but I have seen it)

Check the rubber hoses, they will breakdown inside and collapse. I just went through that with my 52 Chevy truck, couldn't get the brakes to bleed, no fluid or very little, replaced the proportioning valve and same thing, replaced the rubber brake line that goes from rear end to frame cross member, finally got it to bleed right. Hope you get it figured out.

It could be you have a bad master cylinder. I went through 3 new Cardone master cylinders before I gave up and bought an AC Delco master cylinder. I have a picture of the third Cardone master cylinder and part of the seal was sticking out the back when I pulled it off the car. If there is a way to post a picture I could show you. With the good master cylinder the brakes bled very easily and I did not have to hold the button on the proportioning valve.

How are you doing the bleeding? Just the foot brake?

I have had really good luck with the cheapo vacuum pump.

If you find that you can't get any fluid with the vacuum pump then then you at least know it might be a collapsed line.

I went through the same thing
2 bad masters, could not get any pressure to the rear.
never assume on parts these days.