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??

[Gearren]

I neglected to say that I did set the venting clearance in accoradance to Haynes manual several times. You're saying that Haynes is 2x too large a clearance? I'll have to try reducing the clearance.

Years ago I switched the rubber brake lines to stainless steel braided. At that time 83? I also switched to a 19 mm m/c, which made the pedal harder to push, but did little to slow the car any better. This time around, I elected to go back to a stock 17mm and see if I could improve the actual stopping power of the brakes. I can live with alittle more pedal travel, but I need for the car to stop when I want it to. It seems like the pads just don't exert enough force on the discs.

I have been bleeding the system with the 2 man system, which has the 2nd man getting pretty tired. I am using Dot 3 brake fluid (Prestone), could the fluid be a contributor?

I've read some threads about losing the prop valve and installing a "T" fitting, and it seems like an
interesting idea.