'71 El Camino, I had new, rotors, calipers, rear wheel cylinders, shoes, booster, master cylinder
installed. Brakes still spongy and leak down. Could it still have air in the lines or could it be the
proportioning valve?
Thanks, Rick

IF it wasn't bled properly, yes, air in the system is a possibility.

air in the system or the master is NG. Is it a rebuilt or new? could also be spongy from worn out flexible brake hoses.

I don't know how true this is... But Sometimes new master cylinders need to be "broken in"

On my 70 elky... the previous owner didn't drive it much for about a year, and the MC went out.. He replaced it right before i bought it. The new MC was soft too, but after driving it for about a week, it firmmed up.

Worked for me...

It is a rebuilt MC. I'm going to bleed the system this weekend.