My 2000 f150's brakes are soft. I bleed them and they worked great for a week and now their soft again. They don't go to the floor but they come close. When the motor is off you can pump them up and they get hard. It has plenty of brake fluid.
If adjusting the rear brakes doesn't help, check all the steel brake fluid lines, especially where attached to the frame.
One of my trucks, a year older than yours, did the same thing for a couple of weeks. The front to rear line, just forward of the fuel filter, rusted through. The leak initially was very small, and was absorbed by dirt/rust in the frame channel. While it was parked and I was underneath ( with someone else pumping the brakes, I heard it first, then it was easy to pinpoint.
It could be a few things, but check the easy stuff first. Other possibilities are a bad flex line, worn master cylinder, vac leak in the booster, or a faulty HCU. You mentioned bleeding the brakes. Is this because of the soft pedal, or because of a major brake job ?
Sometimes, after a brake job that involves replacing front or rear calipers, or a leak in the front end lines, air can get into the ABS control module.. If so, normal bleeding methods do not always get the air out because the solenoids that pump the brakes can also trap air. The ABS system won't cycle when you do the work in your garage. A dealer can use software in a laptop to simulate ABS function, then bleed the air out. It called a "service bleed". Still, check the easy stuff first.
Thanks for the ideas guys I will try them. I did bleed because were getting softer. But I did replace the rear brake caliper and steal line. The brakes haven't been 100% right since but they act like there getting softer.