Brake issues on 72 burb

[Reeds70]

Hey guys, any suggestions will be appreciated. I am about at my wits end chasing my brake issue. I was driving and headed down an old mountain road on the Blue Ridge Parkway and I felt my pedal get spongy, so I pressed it to the floor and it didn;t stop. I was able to gear down and get it stopped. When I got out the front rotors were so hot they were smoking, anyway, I got it home by a tow. I put it on a lift and couldn't find any leaks or broken lines. I double checked that the rear brakes were adjusted, bled the brakes several times. The pedal still seems weak to me, it stops but I don't think its as good as it was. The only other thing I can think of is the plunger for the proportioning valve is worn out. I can move it with my finger and I thought it should have pressure on it so it would be moved so easily? And I think it would be rare that it would fail? Im having trouble finding a replacement that is AC Delco, so if I have to replace it any suggestions on quality aftermarket. Thanks...Also right as I got it stopped the brake light came on in the dash and hasn't went out since.