Thanks for asking, but I think it's the proportioning valve. I had to walk away from it for a few days. If you know what I mean.

 

Been there, done that, got the T shirt. Sometimes that's just what you need to do. Do you have a line on new one? I know ILT has them.

Pn# - PR102 - 1pc $85.00 ea
1971-77 A-body, 71-77 F/X-body, disc brake proportioning valve -
also for 64-72 disc brake conversions - has correct wire lead.

 

And what was the outcome to this thread??? Was it prop valve? I have same issue. On my second prop valve. After 20 mins of driving pedal is rock hard. Prop valve dripping from rear rubber boot. It's the new one for disc conversion from inline tube.

 

I found a guy here in Ft.Collins who builds and races BB Oldsmobiles. He has 5, including a beautiful '71 442 rated at about 700 hp! He did the final tune on my rebuild, and told me the stiff pedal was a function of the cam and the low vacuum resulting from that grind. The final tuning helped a bit, but it's something I can live with. She's not a daily driver, and the vac does build up after driving a while, so it's a trade-off between a lopey, sweet sounding idle with great mid-range power, and something my wife could live with. (Not that I don't want her driving the blue beauty!)

 

I did not read all of this post so sorry if this has been mentioned but collapsed flex (brake) hoses will give you a hard pedal and very poor braking. Replace all of your brake hoses, 2 in front and 1 in back.