Ohhh, No brakes could be a LOT of things on something that old. I'll assume it has power brakes, vacuum assist. Could be a leak in the vacuum hose from the intake manifold to the vacuum booster, kinda big round chamber mounted to the firewall next to the steering column under the hood.
You didn't mention the brakes squealing so I'll assume it has brake material left on the frt pads & rear shoes. If the truck sat for a while it could be that the brake pistons in the frt disc brakes "Stuck" in the piston bores of the front calipers. FORD played around with molded plastic pistons that actually swelled when in contact with brake fluid, not sure if Your truck would have them or not.
Most of the braking power comes from the frt brakes, but if You loose rear brakes stopping distance really grows. The self-adjusters on the rear brakes may be stuck and the rear shoes aren't contacting the drums.
Also could be a bad master cylinder under the hood leaking fluid when You press down on the brake pedal. Normally that throws a warning light on the dash but on a 24 yr old truck the light may not work. Normally the warning light is the "Emergency brake light".
Also, even if there's brake material left on the frt pads & rear shoes, if the brakes have been used really hard trying to stop a heavy trailer, loaded truck, etc. the linings could be glazed over really bad.
Also on a truck that old the rubber brake lines deteriorate, there's at least one at each front wheel, and at least one running from the frame to the rear axle. When pressing on the brakes the hoses could be swelling, reducing the force the pads & shoes press against the discs & drums with.
Also could be as simple as dust or brake fluid, maybe even some stray grease getting on the brake linings.
I hope You didn't spend $3000 throwing parts at the brake system trying to get them to work. That much $$$ should have replaced everything from the rubber pad on the brake pedal all the rest of the way through the entire system.
This is just like mine except mine is blue.
