4Runner brake problem

I realize that this is a Tacoma forum, but I need another opinion and y'all have been helpful in the past, and I'm hoping you can be again.

In addition to my Tacoma, I've got a 1998 4Runner, 2WD, 3.4, with ABS. It's got what I would call a soft brake pedal.

With the truck off when I step on the brake the pedal slowly sinks nearly to the floor. With the truck on this very much more pronounced and faster.

I have:
Replaced the soft lines on the front brakes.
Replaced the master cylinder twice (the first one was a duralast from autozone, the second is a Centric from Rock Auto).
Bled and bled and bled the system.
Verified and adjusted the drum brakes.

When I turn the key to "on" I don't hear the ABS pump priming, but I do hear it click 3 times - at least I think it's the ABS pump. The clicking comes from a part mounted high near the firewall on the passenger side and has 5 (as I recall) hard brake lines to/from it.

I've tested the brake booster by stepping on the brake with the truck off, then turning it on. With the truck off there is a greater resistance in the pedal than with it off.

I've pulled the ABS fuse and tested the brakes and noticed no change.

I believe that I've either got 3 bad master cylinders in a row (the one that was on the truck when I bought it plus 2 new ones), or there's something wrong in the ABS system. I don't know how to tell which it is, or if it's something else.

What advice do y'all have for me?

Thanks,

Matt