1968 Dart Brake and Booster issues?
It seems like the $35 unit from Summit is sort of weeping out every port, since original installation.
Do you still have the aftermarket P-valve installed?
There seems to be a little sigh/hiss/wheeze from the brake master after it first is depressed.
this is normal.
When you get so many leaks, you have to step back and think about it. I'll bet you a chewing gum wrapper that the flares were done incorrectly, or the lines used were very hard..
Double Flares are hard enough to do correctly and successfully on soft lines by experienced people. If your mechanic did not double-flare your lines, that is a problem. If he did and failed to properly inspect them afterwards, that may also be the problem.
That aside;
The rear brakes from the factory, are limited to doing about 15% of the work. And the P valve works like a hold-back valve, making sure the fronts are well into doing their jobs before the rears get activated.
Furthermore, the rear brakes cannot work until the wheel cylinders actually push the shoes out into drum contact.
Furthermore, if the radius of the shoes does not match the radius of the drums, all the force will go to a small segment across the shoes maybe a half inch wide or LESS with the tires not contacting the road and turning.
If the pedal is high and hard, and has been so since the beginning;
then chances are nothing is "wrong". It might not be optimum, but it probably is not wrong.
As to Proportioning;
My car started out with 235/60-14s on the front and 275/60-15s on the rear. With the KH 4-pistons on the front and 10X2s on the back, NO PROPORTIONING was needed. I simply used one size smaller rear wheel cylinders.
After I switched to 295s, I went back to the larger w/c's.
and done