This is a typical vintage...
This is a typical vintage Mustang braking system. In the old days, Mustang braking systems were downright dangerous because they didnt have the redundancy of the federally mandated, dual-braking systems that came in 1967. For 1965-66, a single master cylinder forced hydraulic pressure to four drum brakes. Optional were front disc brakes that received pressure from the same master cylinder.
We all probably take our brakes for granted. We dont think about brakes until we need them or when they fail us.
When you get right down to it, the physics of braking is taking the kinetic energy of a moving vehicle and transforming it into heat energy. Okay, so what the heck does this have to do with stopping a Mustang? Plenty. When we step on our Mustangs brake pedal, we are forcing brake fluid under pressure from the master cylinder through steel lines and hoses to hydraulic servos (wheel cylinders or calipers) that move brake pads or shoes into the rotating mass that is a brake drum or rotor. The friction material that contacts the rotating iron or steel mass slows the rotor or drum to a stop, which heats up the pad, shoe, rotor, or drum. We are transforming kinetic energy (motion) into heat energy when we apply resistance to that kinetic energy.
This is a master cylinder...
This is a master cylinder for a single braking system. One reservoir and cylinder supply braking pressure for four brakes. These master cylinders fail because the rubber cups and seals wear out or fluid becomes contaminated.
When we get past the theory of braking relativity, our objective is basically one thing: to stop safely and soundly. So what are brakes? What are they made of? What do they do? And how do they do it? Mustangs are equipped with two basic types of brakes: drum and disc. Some have power assist to ease braking effort.
Braking begins with a Mustangs brake hydraulic system. Hydraulics puts fluid to work for us. Hydraulic pressure could literally move the planet. Because we cannot compress fluid, we move it through lines and hoses to the device we need to do the work. When you step on the brake pedal, the master cylinder moves fluid through the lines and hoses to the wheel cylinders or disc brake calipers, which moves the shoes or pads against the rotating drum or rotor. If there is air in the fluid, we compress (squeeze) the air and fail to move the brake fluid effectively. This is why brakes that need bleeding (getting the air out) feel spongy. When there is solid brake fluid between the master cylinder and wheel cylinder/caliper, we have a hard pedal and effective braking.
We bleed the air and contaminated fluid out of a braking system by having someone step on the brake pedal, then opening bleeders at each of the brake cylinders/calipers to remove air. Once all air and contaminated fluid are removed from the system, braking efficiency improves dramatically. Braking systems should be flushed and bled at every brake job. This keeps the fluid fresh and effective. Contaminated brake fluid hurts braking effectiveness and your safety.
 Beginning in 1967, Ford went...  Beginning in 1967, Ford went to the federally mandated dual braking system, which provides braking pressure should the front or rear system fail. This is a dual reservoir master cylinder, with separate reservoirs feeding master cylinder bores positioned in tandem. Stepping on the brake pedal pressurizes separate front and rear brake systems. |  Dual braking systems must...  Dual braking systems must have a pressure-differential valve in order to remain separated, plus inform the driver when theres trouble. This valve is affected by brake pressure difference between front and rear brakes. A loss of pressure in the rear brakes slides the valve piston to one side, closing the switch and illuminating the BRAKE light on the instrument panel. Less than motivated mechanics have disconnected these switches through the years rather than do a differential brake bleeding to center the piston. |  This illustration shows the...  This illustration shows the brake pedal/master cylinder relationship and a simple manual brake setup. |