Here goes.
97 EXP 2WD XLT. 4 wheel disc brakes, 4 wheel antilock brakes. 285k miles. Both my kids have driven it through high school.
The other day, my daughter tells me she hears some scraping when she brakes. Within the last year I put on new front discs/bearings (it's all one unit on mine), as well as pads.
While checking the dual piston calipers, a couple pistons were stuck. I figure 28 years with the same calipers and pistons, I ought to be happy with that.
Here is what was replaced.
Both front calipers
All 4 brake hoses.
Rebuilt rear calipers, as it is a pretty easy job.
New master cylinder (bench bled, as per instructions)

Took it for a drive, and still felt "spongy". When I put temp gun on the rear brakes, they were ambient temp. They never engaged at all. Tried to re-bleed. Nothing.

I get fluid leaking when I loosen both fittings at the master cylinder.
I get fluid leaking when I loosen the line that goes into the ABS unit from the master.
I get fluid leaking from both front bleeder fittings at the caliper.

I get NO FLUID at the rear where the brake line meets the junction that feeds both rear brakes. The brake line from the ABS unit connects to a block that is part of the left rear brake hose. Then from that block, a line goes across the truck to the right rear brake hose. No fluid at all.
I get NO FLUID if I loosen the fitting that is the brake line from the ABS UNIT to the rear brakes.

I get no fluid to rear brakes with pumping. The brake pedal is hard, since I guess both front bleeders are closed, and there is a "clog" somewhere?

I'm going to disconnect the line that connects the ABS unit to the rear, and blow air through it to see if it is clogged. (I got to wait for a helper)

Thanks for any advice and information on this.