Spongy brakes, 914-4 brakes won't firm up??! Options

[Gearren]

I've had it! I have done everything to the brakes on my car, and I can't seem to get them to stop the car with authority. Best I get is a reluctant coast to a stop. Hardly suitable for street use! I have owned the 73 2.0 since new, and have kept it in the garage since 85. Brakes have always been weak, so as I starterd on the quest to revive the car, the brakes were top on the list. I have rebuilt both front and rear calipers, turned the rotors, new pads, new M/C (17mm), bled the m/c, bled the prop valve, and bled the system at least 10 times! I still have weak/spongy brakes! I want to keep the car stock, however I am tempted to remove the prop valve to see if it is the culprit. Any ideas or suggestions would be appreciated??

[lapuwali]

Pad material can make a significant difference. You said "new pads", but not what pads.

Stainless lines don't live forever, either. If they're 20 years old, as I think you meant, then replacing them wouldn't be a bad idea.

As mentioned, improper venting clearance will require not only longer pedal travel, but pumping the pedal should firm it up. If you press the pedal hard and hold it down, it shouldn't continue to sink even if you hold it down a long time, or you have a leak somewhere (perhaps internal). If pumping doesn't help, it's not a venting clearance problem.