Some of the info given here is nuts.
If you can tell the pedal is softer than it was before the work then you have air in lines plain and simple.
Mityvacs absolutely suck for brake bleeding, I don't care what the product says. I have two and have done it with them. They pass way too much air right at your fitting through the threads, you will seem to get air forever and it will not go away.
If you DO have ABS, then as long as ABS never activated during the low pedal problem (it shouldn't even be possible) then the ABS system is transparent to the lines, they are not affected, at least on any I ever worked on. Meaning you bleed like on a normal non-ABS car.
'Brake fluid does not compress. So if you put the cap back on, then you have someone open up a bleeder, push fluid out, tighten the bleeder, and then let up on the pedal, things can break.'
When you let up on pedal the m/c simply draws more fluid from the reservoir. Like the system does all year long in use when brake pads wear. I've been bleeding with cap firmly on for 45 years with not one mishap.
When doing the two man manual bleed if you open the bleeder quickly and all at once, the force of brake fluid spurting out can pull air in at end of squirt so fast you can't see it. The positive pressure pulls negative behind it just as in all pressure pulse phenomena. Better to slowly crack fitting and try to get it closed just before pedal stroke ends, meaning no more than a one inch jet of fluid. Half would be better. If person inside car cannot be relied on to follow direction 100% (not 98 or 99%) correctly, then you cannot do the bleeding right. Exact communication there is essential.