After replacing the rear brake rotors and pads, the brake became soft?

My vehicle listed above has the same problem (Same model with the Caravan and Town and Country), after replacing rear brake rotors and pads, the brake became soft, Google search said air or water in the brake fluid, but the mechanic didn't do this part job, all the brake line is still same as before. I bought new dot 4 brake fluid and replaced all of them, but it didn't work, and until I watched this video, I confirmed the problem is caliper didn't adjust correctly:
https://www.youtube.com/watch?v=7SKXA3CMlP8
It means, all this type caliper brake, after the brake repairing and install them back, the caliper should push the piston to keep the brake rotor and brake rotor has no space there, if you have a big gap there, the brake pedal should use more move distance to move this gap, this is the reason the brake became soft, this type caliper is come with a parking brake. Its design can move the piston when the brake pad becomes thin, but this function could be lost without regular brake maintenance.
I can't find a picture to show this, it's just rotate the piston, it can adjust the piston stop location, make this location is minimal gap between rotor and pad.