Jim Smart
September 1, 2000
This is a typical vintage Mustang braking system. In the old days, Mustang braking systems were downright dangerous because they didn’t 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 don’t 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 Mustang’s 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 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 Mustang’s 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.

Drum Brakes

Drum brakes have been around since the beginning of the 20th century and there have been many variations along the way. Most classic Mustangs (1965-’73) were equipped with drum brakes. All 1974-’93 Mustangs were fitted with front disc brakes. Mustangs after 1993 were fitted with four-wheel disc brakes. This should tell you something about braking technology. Drum brakes are old technology. Front disc brakes have been available as an option on the Mustang from day one.

A drum brake is more complex than a disc brake because there are more parts involved. The wheel cylinder receives hydraulic pressure from the master cylinder when we apply the brakes. Wheel cylinder cups and pistons move outward against the “C”-shaped shoes lined with friction material. The friction material contacts the rotating drum, which is an integral part of the axle and wheel. This friction brings the drum, wheel, and tire to a stop. Three return springs pull the shoes away from the drum when the pedal is released. A star-wheel adjuster between the two shoes at the bottom adjusts the shoes in or out.

Self-adjusting drum brakes employ even more parts. All Mustangs prior to 1994 have self-adjusting drum brakes. The self-adjuster consists of a cable, pivot hook, and return spring, and it only works when backing up and applying the brakes. As the shoes contact the drum while backing up, the pivot hook (tied to the cable, shoe, and return spring) moves away from the star-wheel adjuster, then returns, moving the star-wheel adjuster, thus moving the shoes closer to the drum. When properly maintained, self-adjusting drum brakes work quite well, but they quit working whenever the pivot hook is damaged. They also quit whenever the star-wheel adjuster ceases. A good rule of thumb is to inspect the self-adjuster for proper operation any time you repack wheel bearings or rotate tires. Get familiar with your drum brakes. They can save your life.

The parking brake holds one of the rear brake shoes against the drum to hold the vehicle. We pull on the “T” handle, press the parking brake pedal, or pull up on the handle, which pulls a cable tied to the rear drum brake shoes.

Drum brakes deserve more credit for braking effectiveness than we give them. Drum brakes offer us more braking surface area than a disc brake. They can be very effective, especially at the rear axle. The problem with drum brakes is their effectiveness in some situations. They’re downright dangerous in wet weather. If they get soaking wet in high water, for example, they will not stop a car because water between the shoe and drum causes the friction material to hydroplane, just like a tire does on wet pavement. Water keeps the friction material from contacting the drum and stopping the vehicle.

Drum brake maintenance should include a regular inspection schedule. Pull the drums and inspect them every 5,000 to 10,000 miles. Lubricate the star-wheel adjuster with a thin, high-temperature grease. Examine the wheel cylinder rubber boots for evidence of leakage. If there is brake fluid inside the rubber boot, rebuild or replace the wheel cylinder.

If your drum brakes tend to be noisy, when are they noisy? If they squeak when you’re applying brake pedal pressure at a standstill, there isn’t enough lubrication between the shoe and the brake backing plate. If they squeal during application with the vehicle in motion, the shoes and drums are glazed and should be serviced. Drums and shoes become glazed whenever there has been heavy braking and severe heat. They can also become noisy whenever the drum has been turned too many times and is out of tolerance. Then the shoes have to travel beyond their normal travel distance, contacting the drum unevenly or not at all.

This is the single-piston Kelsey-Hayes disc brake used on the Mustang from 1968-up. The most significant change to this brake came in 1970 when Ford gave it a larger spindle. Look for the large-spindle disc brake on 1970-’87 Mavericks and Comets, and 1975-’80 Granadas and Monarchs. Parts and complete systems are available from Master Power Brakes as well.

Disc Brakes
There has always been a more sophisticated, upscale attitude about disc brakes. For one thing, disc brakes outperform drum brakes by a wide margin. Disc brakes don’t fade. Disc brakes are simple compared to drum brakes. Disc brakes tolerate hard braking better than drums.

To understand how a disc brake works, we have to understand the design. First, we have either floating or fixed caliper disc brakes. Then we have one-, four-, or six-piston calipers. Disc brake caliper pistons do the same thing wheel cylinder pistons do in a drum brake. They transfer fluid pressure to the friction material that stops us. The more pistons we have in a disc brake, the better. First generation Mustang disc brakes (1965-’67) have four-piston disc brake calipers. This means we have four pistons transferring pressure to brake pads on both sides of the rotor. A four-piston disc brake applies more uniform pressure to a brake rotor.

From 1968-up, Ford went to a single-piston, floating caliper disc brake that became a mainstay on Fords for many years. One large piston transfers fluid pressure to brake pads on both sides of the rotor. Because the caliper floats, pressure is applied to both sides of the rotor. Simply put, the caliper squeezes the rotor.

This is a cross-section illustration of a vacuum power booster at rest. Vacuum from the intake manifold is closed off from the booster chamber when the brake pedal is at rest.

How Power Brakes Work
What exactly are power brakes and how do they work? All Mustangs produced since March 1964 have been equipped with two basic kinds of power brakes. Most common is vacuum-assisted. Vacuum assist helps us apply the brakes, reducing pedal effort.

Vacuum-assisted (or boosted) power brakes were used in Mustangs until 1996 when the huge 4.6L SOHC and DOHC V-8 engines filled the engine compartment to capacity, making it impossible to fit a vacuum booster between the engine and inner fender. Thus, Ford went to hydroboosted power brakes, which consume less space. Hydroboost gets its pressure from the power steering pump.

Vacuum-assisted power brake boosters get their vacuum from the engine’s induction system (intake manifold). When we’re running a really radical engine, intake manifold vacuum suffers, making it necessary to use an electric or engine-driven vacuum pump to help the power booster.

Hydroboost is nothing new. It’s been used in Lincolns, Cadillacs, and other types of luxury cars for ages. It works on the same basic principle as power steering. We use hydraulic pressure from the power steering pump to help apply the brake pedal. This is the same basic principle as the Bendix power-assisted steering common to 1965-’70 Mustangs. Move the steering wheel and you “tell” a control valve to send hydraulic pressure to one side of the steering ram or the other. This helps you steer. Same idea with hydroboosted power brakes. Touch the brake pedal and you tell the hydroboost to apply pedal pressure.

Did you know you can install hydroboost power brakes in your classic Mustang? Just go to the Lincoln parts shelf for answers. However, hydroboost power brakes can be more high maintenance than vacuum assist.

What Is Rotor Seasoning?
According to Baer Brake Systems, disc brake rotor seasoning is a vital first step with new rotors. Think of it like you would the molding of a baby’s personality because you will certainly be stuck with the results if you don’t handle it properly. Seasoning new rotors is important because it molds the molecular structure of the iron/steel. When we properly season new rotors, we’re burning the factory machine oils off of the surface and establishing a pad/rotor marriage.

The most complex break-in task is relieving the internal stresses within the iron/steel. If this is hard to understand, think of it like you would pouring water into a glass of ice. The casting and cooling of the iron leaves the material with internal stresses you don’t need. How to relieve stress? Here’s what Baer suggests.

  • Drive the vehicle for five or six days without heavy braking. Normal braking helps relieve internal stresses because we’re cycling the iron from cold to hot and back to cold again. Baer adds that zinc-plated rotors take even more time.
  • Find a safe location where you can get those brake temperatures way up.
  • You want to gradually increase brake temperature with progressively faster stops. Begin with 60 to 70 mph stops as you would in normal driving. Do not slam on the brakes.
  • Next, perform four medium effort partial stops from 60 mph down to 15 mph. Follow this with five minutes of highway driving with little or no braking.
  • Next, perform four medium to hard effort partial stops from 60 mph down to 15 mph. Follow this with 10 minutes of open highway driving, with little or no braking, to allow the rotors to cool.
  • Park the vehicle and allow the brakes to cool overnight. You’re halfway there.
  • Return to the safe location and get your brakes up to temperature. Make sure the brakes are warm. Repeat the same procedure you did the day before. First, four medium pedal effort stops from 60 mph down to 15 mph. Again, follow this with five minutes of highway driving for brake cooldown.
  • Again, perform four medium to hard effort partial stops from 60 mph down to 15 mph. Follow this with 10 minutes of open highway driving for another brake cooldown.
  • Next, get your Mustang up to 60 mph and do six very hard partial stops down to 15 mph. Brake rotor temperature should be 900 to 1,100 degrees F. Baer sells a special paint that indicates brake temperature. Follow this sequence with 10 minutes of open highway driving for brake cooldown.
  • Allow brakes to cool overnight.
  • With the rotors properly seasoned, we’re ready to bed the pads. Bedding the pads is important because it marries the pad to the rotor. The friction material in semi-metallic brake pads is held together by an organic binder. As the pad gets hot, the binder tends to boil and burn. As this occurs, the friction material makes better contact with the rotor. Racing pads, like those we see from Performance Friction, lay down a layer of carbon on the surface of the rotor. This is necessary for these pads to perform well.
  • Always follow this procedure exactly. No short cuts. This process of rotor seasoning and pad bedding allows pad and rotor to mate comfortably for a pleasant service life that will serve you well.
  • What Is A Proportioning Valve?
    A proportioning valve controls brake pressure to the rear brakes. When we apply brakes, we want strong brake pressure in front, and less pressure in back because the front brakes should always be the dominate binders. If the rear brakes come on too soon or with more aggressiveness than the front brakes, control of the vehicle can be lost. Remember: front brakes first, then the rears.

    The Bloody Truth
    Do you know how to properly bleed brakes? Baer Brakes stresses the use of the best brake fluids available. Most recommended is Performance Friction’s Z-Rated fluid (#90016). If this isn’t available, opt for Ford’s HD #C6AZ-19542-AA brake fluid instead. What Baer likes most about these fluids is the way they’re packaged. Plastic containers allow moisture to get into brake fluid just sitting on the shelf. Use brake fluids sold in cans for best results.

    When you fill the master cylinder with brake fluid, always take it easy and don’t aerate the fluid. Begin bleeding with the farthest brake from the master cylinder. You need clear tubing and a clear plastic or glass jar. Fill the jar just enough to submerge the end of the plastic tube. Have a partner apply and maintain pedal pressure. With the clear hose snug around the bleeder, open the bleeder with a wrench and observe fluid flow. Have the partner gently pump the brake pedal until all air and dirty fluid has escaped. Continue this process until you have clean fluid flow, free of air bubbles. Always close the bleeder with the pedal at the floor—this eliminates the risk of air being drawn back into the system.

    Move along to the left/rear brake and repeat the process. Check the master cylinder for fluid and never allow it to run dry. Move the right/front brake and bleed it the same way. Then bleed the left/front brake.

    When the bleeding is finished, you still have work to do. Take a block of wood or plastic hammer and tap the brake calipers, dislodging any trapped air bubbles. Bleed the front brakes again, beginning with the right, then left. Make sure all surfaces are clean and free of brake fluid. Use a brake cleaner for this purpose. Brake cleaner has a very high evaporation rate and dries quickly.

    After you have driven the vehicle for a couple of days, we suggest going back and bleeding the brakes a second time to ensure all air has escaped. Follow the same procedure for best results.

    Types Of Brake Pads
    Stainless Steel Brakes tells us there are three basic kinds of brake pads: Organic, semi-metallic, and full-race. Which one should you choose and why?

  • Organic pads are the most common brake pad designed for ordinary use. In regular use, without abuse, these guys should go 30,000 to 40,000 miles.
  • Semi-metallic pads are better than organic because they handle and stand up to heavy use better. Because semi-metallic pads create more brake dust than organic pads, they dirty wheels more quickly.
  • Full-race pads are designed to endure high-heat, severe-duty service. These pads work better when they’re hot—on the racecourse or a canyon road. In regular street use, they’re noisy. They chatter when they’re cold. And they will stop you aggressively in most situations.

    Brake System Question & Answer

    Q: Why do we coil steel brake lines in some locations?
    A: Stainless Steel Brakes says to give the brake system plumbing flexibility. Coiling allows the lines to flex with fluid pressure inside, plus any movement of body and chassis.

    Q: How many different kinds of brake fluid are there?
    A: There are three basic brake fluid requirements mandated by the U.S. Department of Transportation (DOT). They are DOT 3, DOT 4, and DOT 5. DOT 3 and DOT 4 are glycol-type fluids. DOT 5 is silicone based. Because DOT 3 and 4 are mineral based, they tend to absorb moisture. When brake fluid absorbs moisture, the moisture boils when placed under high pressure. The moisture boils within the brake fluid from the heat of braking pressure. When it boils, it creates air pockets in the fluid. Silicone-based DOT 5 does not absorb moisture. Therefore, it is a more stable fluid, especially during hard use. Never mix DOT 3 and 4 with DOT 5.

    Q: Why do we bleed a master cylinder off the car?
    A: Baer Brake Systems says we “bench bleed” a master cylinder to ensure the piston has purged all of the air in the bore. If you bleed the master cylinder on the vehicle, the brake pedal doesn’t always move the piston the full length of the bore, which can leave some air behind. When we bench bleed a master cylinder, we want to route the fluid back into the reservoir. Bench bleeding kits are available for this purpose. New and rebuilt master cylinders also sometimes include bleeding tubes for this purpose.

    Q: Which is better? Single-piston or multi-piston disc brakes?
    A: Without question, a multi-piston caliper is better, according to Stainless Steel Brakes, because four or six pistons apply uniform pressure on pads and rotors, giving us better braking efficiency. Single-piston appears to be more popular because people perceive it to be a better brake. From a maintenance standpoint, the single-piston caliper wins because it is simple, with fewer parts.

    Q: Why do disc brake rotors warp?
    A: Baer Brake Systems tells us disc brake rotors warp because they’re not properly seasoned to begin with. Simply put, once a disc brake rotor warps, the battle is lost, even with a trip to the brake lathe. Baer strongly suggests properly seasoning brake rotors when they’re new, which prevents warpage.

    Q: Which is better? Front disc/rear drum, or four-wheel disc?
    A: According to Stainless Steel Brakes Corporation, four-wheel disc brakes provide the best stopping power in most applications. But performance depends largely on the vehicle, SSB says. If you’re going to road race, four-wheel disc brakes experience less fade than front disc/rear drum applications. Drum brakes are prone to fade in severe duty situations. Street vehicles that are light in the rear end benefit very little from the installation of rear disc brakes. The nice part about rear disc brakes is simplicity and ease of maintenance.

    Q: When we say “Stainless Steel Brakes,” what does the name mean?
    A: The company, Stainless Steel Brakes Corporation, was founded to improve Corvette and Mustang four-piston front disc brakes more than 20 years ago. According to Stainless Steel Brakes, disc brake pistons were chromed to help prevent corrosion, but chromed disc brake pistons never did perform well. Chrome would flake off, cutting seals, causing leaks, and sticking pistons. Stainless Steel Brakes concluded a stainless steel piston would perform better. The company and its idea have been something of a legend ever since.

    Q: Why do we turn brake rotors and drums during a brake job?
    A: Because we want to give new pads and shoes a uniform surface to mate with. If you don’t turn rotors and drums, the pads and shoes will have a hard time wearing into an already glazed surface. Baer Brakes advises against turning disc brake rotors because you compromise the structural integrity of the rotor. In taking metal away on the brake lathe, there’s less metal in the rotor, which can lead to warpage. If you have a scored or warped rotor, we advise replacement if the scoring is deep. Always ask the machine shop to cut the rotor as minimally as possible. All we want to do is eliminate any irregularities and give the new pads/shoes a rough surface in which to seat.