Re: '73 Mustang Brake Problems
being a 73, It has dual circuit brakes. If you look in the master cylinder, it probably has a partition in the reservoir, (Front and rear brakes) hence the reason you cannot find a proportioning valve.
From the sound of it, you know what your doing in the bleed.
I would be looking at two things... Rear brake adjustment and the front calipers.
If you adjust the rears up too tight, then they will come on too aggressivley too quickly.
the front calipers could be seized.