When we back up and hit the brakes, the brakes begin to fade until no brakes. If we drive forward only, no problem. The brakes only fade after hitting the brakes when in reverse. If we then turn off the engine and let it sit for awhile, the brakes come back. Also, there have been a couple of times where when turning the wheel all the way (back and forth, such as parking) that we hear a loud "Clunk" (from the front end?), And then the brakes began to fade. But the brakes do consistently fade every time after backing up and using the brakes. No resistance at all on the brake pedal.
All mechanics know that Ford had some really bad design problems with their brakes throughout the '70s, '80s, and into the '90s. Rather than trying to diagnose the specific problem or combination of problems over a computer, it's probably better to start with the basics since they can be responsible for some of the symptoms you're describing.
First of all, Ford mounts their front calipers with a couple different types of wedges that make it impossible for them to slide sideways freely. Rust and dirt buildup aggravate an already bad problem. Under light braking, only one of the two pads will apply. After hard braking, the caliper is not free to slide and release so one pad stays in contact with the rotor and can overheat leading to one type of brake fade. At first the brake pedal will feel firm and high but the truck just won't want to slow down as quickly as normal. If that gets bad enough, the brake fluid can start to boil leading to a different type of brake fade. Moisture in the brake fluid boils and vaporizes. Those vapor bubbles compress so the brake pedal feels low and mushy. That type of fade will not occur when the brakes are still cold so that can be eliminated if the problem occurs right after starting the truck.
The place to start on the front is by removing the wheels and using a large flat blade screwdriver to pry the pistons back into the calipers. They might go in rather hard but if you can not move them at all that way, the pistons are likely sticking. Both calipers should be replaced in that case. Many do-it-yourselfers use a c-clamp to force the pistons in but if you HAVE TO do it that way, you got junk. Dirt or rust can build up on the pistons after they extend out as the self-adjusting feature as the pads wear down. Pushing the pistons in runs that crud under the square-cut seal and leads to the sticking. If you can't force a piston in with a screwdriver, there's no way it's ever going to release on its own when there is no pressure causing it to retract.
Next, remove the calipers from the mounting knuckles and look at those slides and wedges. How any engineer could expect those mounting systems to work is a mystery, but rust and dirt makes the problem even worse. Run a flat file over all of those surfaces on the calipers and on the mounts. You don't have to shine them up; all you're after is there should be no high spots of rust or dirt buildup that can impede the calipers' ability to slide. Once everything is clean, coat all of the surfaces with high-temperature brake grease. "Rusty Lube" is one trade name I'm familiar with but there are many others. They all contain molybdenum disulphide that will not travel or burn off. Don't use any type of axle grease. That will burn and form a crust that restricts free movement. When you reassemble the calipers, check them for free sideways movement. On all other cars and trucks you can move them very easily with one hand. That insures even pad wear. On Ford trucks you'll need a large hammer to make them move but you should be able to make them slide.
Next head to the rear brakes; there's lots to check. First be sure the shoes are in the correct location as it isn't exactly obvious. The shorter linings go toward the front of the truck and the longer linings go toward the rear. There are a lot of things that will prevent the shoes from self-adjusting properly. Ford has way more than their share of sticking parking brake cables. When they don't fully release, no self-adjustment will take place. That can leave excessive clearance between the shoes and drums which causes the brake pedal to travel further than normal. A quick clue to misadjusted shoes is the brake pedal will get higher and harder if you pump it rapidly a few times, then hold pressure on it. As soon as you release the pedal, then reapply it a few seconds later, it will be low again. Pumping rapidly allows the master cylinder to take a new bite of fluid before the rear shoes have time to fully retract. With each stroke, the shoes move out a little further until they contact the drums. Manually readjusting the shoes will solve that problem for a while but the real fix is to address the cause. To check for sticking parking brake cables, first look at the tops of the shoe frames. They both must be contacting the large anchor pin on top of the backing plate. Next, use your thumb to push the parking brake strut rod, (between the two shoes), against the anti-rattle spring pressure. That bar should move a good 1/8". If it is tight or the shoes aren't against the anchor pin, use a pry bar to force the parking brake lever rearward. That will tug on the cable to release it. On many Ford trucks, the passenger side cable comes in from the back and the lever is on the front shoe but the procedure is the same. Once the cable is fully released, that strut rod will have some free play. Most likely the shoes will need a lot of adjustment. There should be just the slightest hint of drag on the drums when you turn them by hand.
Don't overlook the possibility of a lining rusted off the shoe frame and going around with the drum. That's one thing that was not common but it can happen on any vehicle.
When you look at the sides of the shoes you will see three "lands" on each one. Those are raised surfaces, often bent over tabs that have mates on the other side too. Those are the areas the shoes ride on where they contact the backing plates. Pull the shoes away from the backing plates and inspect those areas to see if grooves are worn in them from the shoes sliding back and forth all these years. Deep grooves can cause the shoes to hang up and not move out, or in rare cases they can get caught and not release. If those grooves are not too severe, they can sometimes be ground or filed down but if they're real deep, the backing plates must be replaced. This too pertains to any car or truck with drum brakes. This isn't just a Ford issue. Put small dabs of brake grease on those six spots on each backing plate. That will also eliminate a harmless but annoying squeak when releasing the brakes.
Look for a height-sensing proportioning valve in the steel brake line near the rear axle. Most trucks and minivans use them because there can be such a wide range of loading on the rear. The valve will have a link attached to the rear axle or to the leaf spring. When the truck is heavily loaded, that valve allows a higher percentage of fluid pressure to go to the rear brakes so they work harder. When the rear is lightly loaded, that valve limits the amount of rear fluid pressure to prevent rear wheel lockup. If that link is broken or disconnected there won't be enough braking with a heavy load. That should not change the way the brakes feel when backing up but it can cause the brake pedal to feel different during subsequent pedal applications.
Finally, keep in mind the master cylinder can easily be damaged by improper bleeding procedures. Crud and corrosion build up in the bottom halves of the two bores where the pistons don't normally travel. During pedal bleeding with a helper most people run the pedal all the way to the floor. That runs the lip seals over that crud and rips them. That isn't a problem if the master cylinder is less than a few years old, but on older vehicles like the '88 model I drive, that is likely to cause damage. Sometimes the pedal will go to the floor right after the bleeding procedure is done but more often the low pedal will be intermittent for a few days before the total failure takes place. At first a little nick will still seal sometimes, but as the tear continues, internal leakage prevents any pressure from being developed and the pedal goes to the floor without ever applying the brakes. The only cure for that is a rebuilt master cylinder.
Be very careful too when filling the brake fluid that you don't get the slightest hint of petroleum product in the fluid. That includes power steering fluid, automatic transmission fluid, engine oil, axle grease, and penetrating oil. Any of those will cause rubber parts to swell. The symptoms do not match what you described, but it's worth mentioning anyway. The first clue is usually smoking front brakes that don't release until they cool down. The next clue is the rubber bladder seal under the reservoir cap will be ballooned up and mushy and you won't be able to reinstall it properly. The same thing happens to the lips seals. They grow past the return ports and prevent the brake fluid from returning. That trapped fluid keeps the brakes applied so they get real hot, then the fluid expands and applies the brakes even harder. The only proper fix for that is to replace every component in the hydraulic system that has rubber that comes in contact with the brake fluid, and you must flush and dry the steel lines. That's a real expensive repair even when you don't have anti-lock brakes. A thousand dollar ABS control unit can double the cost of that repair.