Holley 1920 issues

Hang on there,with the brakes. Yes its true the pedal should come back to park itself. And yes its true that insufficient rear brake shoe adjustment will require more pedal travel to affect a stop. However, an incorrect rear shoe adjustment will not cause a pedal parking issue. This is because the rear shoes are returned with the return springs, sending the fluid back to the master, helping to lift the pedal,and expose the compensating port, where excess fluid will be dumped directly into the reservoir. The only thing that could prevent this is if something in the emergency brake system was hanging up, or if rust build up in the cylinders was preventing the pistons from retracting.This is why in post #3, I recommended to isolate the front from the rear. If you have a hard,non-spongy, pedal, or can get one with multiple stabs of the pedal, then it's highly unlikely that there is air in the system. Multiple stabs usually indicates a problem in the rear adjustments.
Now, getting back to the booster.
If after adjusting the rear shoes, or isolating the fronts, you still have a pedal that is not returning, then likely the pushrod is too short. I'm talking about the rod that goes from the top of the pedal to inside the booster. There is a giant spring inside the booster, who's job it is to return the control valve, and the diaphragm, and the pedal to a parked position. The first two are unaffected by a too-short pushrod length, but the pedal is. This is best done last, after the rear shoes are known to be correctly adjusted, and also after it has been proven that there is no air in the system and the pedal is hard with one pump that does not go to the floor.
It is somewhat easy to make the rod too long. If it's only a little too long, then the compensating port inside the M/C can become in-operative, resulting in an ever falling pedal, as the pads and shoes wear out. If the rod becomes much longer than this, it becomes possible to break the control valve inside the booster, rendering the booster junk. The correct adjustment would have the system not doing the above, and also not slamming on the brakes for you at the slightest touch; but having nice progressive boost assistance with increasing pedal pressure. This makes smooth stops a joy.
The pushrod adjustment has a baseline measurement, but I am unable to find it. Barring the spec, it would be a matter of trial and error. The adjustment is done by sliding the booster back far enough to expose the adjusting system. Remember this should be done last, and easy does it. It may require multiple tries.