Rear brakes not working

What is the condition of the suspension's ride height? Has it been altered, as in a lift kit was added?

Dandy. People are not aware of all the problems and potential legal issues they cause when they alter ride height.

All vehicles use a "proportioning valve" that limits how high brake fluid pressure to the rear brakes can go under moderate to heavy braking. Trucks and minivans can have a wide range of loading variables, from no load to heavily-loaded, and no single proportioning valve can be calibrated to meet those variables. Instead, they use a height-sensing proportioning valve at the rear axle, with a link between the body and the axle. When a lift kit is installed, it makes it look to that valve as though the truck is way beyond very lightly loaded. The valve restricts brake fluid flow to the rear to prevent easy rear-wheel lock-up. This is not a concern for you as there is no lift kit, (thank you), but it is still a good idea to look at that valve. In particular, check if the link is broken, disconnected, or rusted off, and the valve is sitting in the lightly-loaded position.

Another way to approach this is to loosen the brake line at the master cylinder and watch for fluid running out when a helper presses the brake pedal. A half a turn on the soft metal line nut is enough. Be sure to tighten the nut before hollering to your helper that it is okay to release the pedal so air does not get sucked in. If air does get pulled in, do not panic. You can have the helper slowly push the pedal, never ever more than half way to the floor, while you open and tighten the nut again. They should push the pedal so slowly that it takes about twenty seconds to push it half way to the floor.

You can do the same thing yourself if you think air got sucked in by the line nut. In fact, this is my trick for never having to bleed at the wheels when replacing a master cylinder. Push the pedal very slowly half way to the floor. That pushes brake fluid down the lines but gives air bubbles a chance to float back up. Now release the pedal very quickly. The fluid rushing back will wash the few remaining bubbles into the reservoir with it.

If you cannot get fluid from the rear brake line at the master cylinder, the master cylinder is suspect, but that is highly unlikely in this case. That would result in no fluid pressure in half the system, and that would turn on the red "brake" warning light on the dash. You would have a low or mushy brake pedal too. You should have a combination valve on the frame rail right under the master cylinder too. That is where the pressure differential switch and valve live that turn on the warning light. Blockage in that assembly is very uncommon, but it is another good place where you can loosen the steel line going to the rear wheels to see if fluid comes out there. It is more important here to be careful that no air gets sucked in if your helper releases the brake pedal before you have that nut tight. The only sure way to get that air out is to bleed at one of the rear wheels.

Up to this point, none of my wondrous story pertains to a truck with anti-lock brakes. Hydraulic controllers seem to be the first thing people suspect when there's a lack-of-fluid-flow problem, but they really do not cause that much trouble. You can do the same steps of opening the lines at various places and looking for fluid flow, but now sucking air in becomes a much more serious concern. If you have rear-wheel-anti-lock brakes, (RWAL), which were real common from the late 1980's to mid 1990's, there is just a small dump valve on the frame rail under the driver's seat. Problems with those are just about unheard of, so look other places first. For air that gets sucked in, just bleed at a rear wheel like that valve was not even there. The concern is when you have four-wheel-anti-lock brakes which is pretty common on all vehicles today. About half of the systems will not let air be bled from two chambers unless you command the computer to open them with a scanner while you are bleeding. Air trapped there will cause a low and mushy brake pedal with an elusive solution. With this system you may not have the rear height-sensing proportioning valve, then finding a place to open the line becomes a little more difficult.

Consider loosening the line right at the rear caliper to look for fluid flow. If you have it there, especially after multiple pedal strokes, it's not unheard of to find air in the caliper and a blocked bleeder screw. Bleeder screws come from the factory with rubber caps to keep water and dirt out of the center hole. When a bleeder screw gets rusted tight, it is because water went down the center hole, then into the threads. Dirt can get in there too and block it. I have had quite a few that I have had to drill through the dirt, then wash it repeatedly to open them up. The trapped air in the caliper will compress and prevent the caliper from applying that brake, and the blocked bleeder screw makes it impossible to get that air out.

I have never heard that and would be pretty disappointed if that were true. Detecting problems and setting diagnostic fault codes is done to aid in knowing which circuit or system needs further diagnosis. It is the beginning step for the mechanic. The brake system is one of the major safety systems, and there can be nothing designed in that would inhibit that system from functioning properly. To say setting a fault code will disable half of the brake system is pure nonsense. Try explaining that to the lawyer or insurance investigator in the courtroom. "I caused the crash because my truck decided to disable my brakes". That would go over like a lead balloon. You can be sure Ford would be answering to NHTSA.

Along those lines, it is worth mentioning that all anti-lock brake systems are an add-on option to the vehicle's base, or "foundation" brake system. While it is true ABS systems can cause a new variety of problems, it is important to understand that even with a total ABS system failure, the base brake system is still there and completely functional. You have the same stopping power as if that vehicle was built without ABS.

Another point of confusion among too many people is they think when the engine stalls and the power steering pump stops, they have no steering at all. People have been known to coast into trees, ditches, and other cars because they did not even try to steer. We read comments to that effect here a few times per year. Same with power brakes. Every vehicle with a vacuum-operated power brake booster will give a minimum of two power assisted pedal applications after the engine stalls. After that it can take two men and a boy to push the brake pedal on most GM vehicles, but the brakes are still there. Some Chrysler products from the early 1990's got their power brake assist from pressurized brake fluid from the ABS system. If you turned the ignition switch off, you would still get well over forty brake pedal applications before that pressure was depleted. If the engine simply stalled, and the ignition switch was still on, you would have unlimited power assist until the battery ran drained. That is plenty of time to safely bring the vehicle to a stop.

I should point out too that with a loss of power steering assist, you likely will not even notice that at highway speed. Power steering is needed more as vehicle speed slows down, and most when standing still.