Basic brake lesson. Brakes work on the principle of the incompressibility of liquids.
Your master cylinder is nothing more than a piston with rubber seals that push brake fluid down a small line which causes other pairs of pistons that are attached to the brake shoes on each wheel to move away from the each other (the wheel cylinder is just a steel tube with two pistons, two seals and a spring to keep the pistons apart, the fluid comes in from a hole in the side between the pistons) .
Now if you have a hole in the line or a leaky master or brake cylinder (usually a leaky cylinder is caused by deteriorated rubber seals), instead of the fluid going down the line and making the pistons ,move apart, the fluid leaks out. When you have a bad wheel cylinder or a hole in the breke lines, you can pump up the brakes, but it will steadily go to the floor again no matter how much brake fluid you put in the master cylinder. A bad master cylinder will usually show as having no brakes at all. A part must be either rebuilt or replaced before you will have brakes.
Now, if for some reason the master cylinder has just gotten low (usually this happens because the brakes wear and more fluid is needed to push the wheel cylinder apart far enough to rub against the drums) you can add more brake fluid and pump the brakes several times to get the brakes to hold firm. Often it is called a "spongey" pedal caused by air in the lines.
The solution is simple. Bleed the brake lines .
To bleed the lines, add fluid to the master cylinder, chock the wheels firmly, start the engine, and have an assistant in the driver's seat to press the brake pedal down while you open the bleed screw at ALL wheels, one at a time, starting with the one farthest from the master cylinder and eventually getting to the one closest. The process is simple.
Yell, "PUMP!" to the assistant, (the assistant pumps the brake pedal),
yell "HOLD!", (the assiatant holds the brake pedal down),
you open (loosen) the bleed screw as the assistant holds the pedal down (brake fluid and air will come squirting out of the lines and the brakes will then go all the way to the floor),
you close (tighten) the bleed screw
then do it again and again until no air comes out of the lines (you'll know when air and when only fluid comes out) and move to the next wheel cylinder and do it until all wheels have been bled of all the air in them. Don't forget to keep adding fluid to the master cylinder.
It WILL be messy unless you have a piece of tubing that just slips over the end of the bleed screw to route the fluid into an old jug or something.
It sounds like a lot but you could have the entire truck done in 15 minutes or less.
The way that a vehicle's brakes are charged at the factory is that an airtight fitting is placed over the master cylinder and then ALL of the air is vacuumed out of the system (the vehicle is fully assembled). Then the vacuum valve closes and a brake fluid valve is opened and the whole system charges in about 5 seconds. No mess, no spillage, no air.
