If a new master cylinder was installed, did it get bench bled prior to installation? This needs to happen to get all the air out before installing and hooking up the lines.

Was the brake system bled properly after all was replaced? Start with the rear right, then the rear left, then the front right, then the front left until air is purged from the lines.

Did the rear brakes get adjusted properly? With rear wheels in the air, adjust the rear brake adjuster until wheel cant spin, then back off adjuster until wheel spins with minimal drag. Repeat on other side.

These things will cause mushy pedal. Get a Haynes or Chiltons manual and it will tell you how to check and do these things.

Air in your HCU could, but unlikely be the cause. If he removed it during you brake job (he shouldn't have), then yes it may be the problem.

I've been told but I have my doubts that if you drive down a gravel road and stomp on the brakes and cause an ABS event that it can get some of the air out of the HCU and into the lines and then bleed the brake system after that. I would personally not trust this method and just get it bled by someone with the HCU bleeding tools. If your ABS is non fuctioning then your like me and have an annoying ABS light on the dash. My issue is a bad ABS Module. I ain't paying for a new one, even from a wrecking yard.

I had a mushy pedal and it turned out to be a bad rear self adjuster. My pads on the left side never made contact with the drum. It made my vehicle pull to the right side when braking. I kept thinking it was my front brakes. After spending money on pads and calipers I found in a forum that rear brakes can cause pull. All better now.

Without ABS working it will brake like there is no ABS. Just like the old days.