How To: Choose and Bleed Brake Fluid by Marlon Mitchell
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It’s long been said you can move the world with the power of hydraulics. Large jetliners use hydraulics to control huge surfaces on the wings and tail to steer them in the air as well as on the ground, and bring them to a safe stop, just like Mustangs.

Malcolm Laughead, one of the original founders of Lockheed-Martin, invented hydraulic brakes in 1918. They’ve been in regular use on automobiles since the ‘20s, practicing the simple process of placing fluid under pressure to transfer braking from the brake pedal to the brake shoes or pads.

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Fords didn’t get hydraulic brakes until the ‘30s due to cost consideration. At the time, mechanical brakes cost less to produce and were less complicated to manufacture, but they weren’t as safe. As automobiles became faster and more mainstream, more and more were fitted with hydraulic brakes.

Hydraulic braking systems didn’t change much for many years after their inception. Drum brakes were common on Fords until the early ‘60s. Ironically, the Mustang had optional disc brakes right from the start when a lot of other Ford products didn’t.

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Brake Maintenance is something we don't take seriously until it's time for major service. Brake components need regular maintenance to keep them like new. Because brake fluid absorbs moisture and other contaminants, it can be hard on steel and iron components. Water is heavier than mineral-based brake fluid. As a result, it settles at the bottom of master cylinders, brake lines, calipers, and wheel cylinders, creating corrosion.


Whether your Mustang has disc or drum or a combination of the two, ensuring contaminant and air-free fluid is universal. Your Mustang must have clean brake fluid that is free of dirt, moisture, and other elements to keep the braking system working at peak efficiency.

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Mustangs prior to '67 used a single braking system, which means all four brakes are on one master cylinder. This is the larger disc-brake master cylinder for '65-'66 Mustangs with front disc brakes. With front disc brakes, there is a proportioning valve between the master cylinder and the rear drum brakes to limit brake pressure to the rear. You want the front brakes to apply first, hence the need for a rear drum-brake proportioning valve.

Brakes and steering are two of the most important systems on your Mustang because they are critical to safety. We pay all kinds of attention to our engines because we don’t want to wind up stranded if the darned thing quits. Yet, we pay attention to the brakes only when there’s a problem. A spongy pedal, brakes that pull to the right or left, unusual noises, or complete brake failure get our attention. Then we wonder why it happened and how to fix it.

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Dual braking systems became federally mandated for '67 models. It consists of two independent braking systems operated from a single master cylinder with tandem bores and two reservoirs. The two systems are separated by a distribution block.

Today, most classic Mustangs are driven infrequently because many are weekend pleasure drivers or trailered show cars. But brake maintenance remains the same whether you drive 15,000 miles a year or a thousand. Brake systems need the same kind of maintenance under all kinds of driving conditions, whether we use them or not. Although brake fluid sits inside steel lines, reinforced hoses, and iron/aluminum castings, it still absorbs moisture and other contaminants from the atmosphere.

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Pre-'67 Mustangs can be converted to dual braking systems with a minimum of fuss. Master Power Brakes offers easy bolt-on kits with power boosters, master cylinders, and distribution blocks you can install in a weekend.

Brake maintenance should be treated just like engine tuning, sloppy steering, or a hole in the muffler. The brakes need attention before problems become threatening. Did you know your brake fluid needs to be changed periodically, just like engine oil and transmission fluid? Did you know mineral (glycol)-based brake fluid can absorb condensation and contain as much as 2-percent water after one year? And if you never change the brake fluid, you’re looking at 7-8-percent water over a period of years. This means potential brake hydraulic failure the next time you slam on the brakes.

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Here's an inside look at the dual-reservoir master cylinder. The smaller front reservoir serves the rear brakes in a front disc-brake application. Rear drum brakes don't require as much reservoir. A larger rear reservoir serves the front disc brakes because they require more fluid capacity. Drum-brake master cylinders have same-size reservoirs. Notice the rough iron casting, which contaminates brake fluid over time because it retains contaminants. Moisture in the brake fluid also attacks the iron casting.

Glycol-based brake fluid absorbs moisture naturally from the air. Think of it as a natural sponge, absorbing moisture while sitting on the garage shelf. It has a shelf life of about one year, then you need to recycle it responsibly. Inside the system, brake fluid takes on moisture through grain boundaries of steel lines and iron/aluminum castings and through pores of rubber and steel-braided hoses. There isn’t a perfectly sealed system. If you live in a humid climate, the problem is only worse.


This is the dual braking-system distribution block with a warning switch. Whenever brake pressure loss is experienced in the front or rear system, a sliding piston inside the block closes the switch to turn on the warning light on the instrument panel. That's a non-adjustable disc-brake proportioning valve to the lower right.

For several years, I’ve contributed How-To write-ups to numerous publications, namely Mustang Monthly. With the help of photographer/writer Jim Smart, these articles have proven helpful to countless readers and enthusiasts. The photos below show all took place at my shop, Marlo's Frame & Alignment.

Which Brake Fluids?

The marketplace is inundated with different types of brake fluid. Which should you use and why? The most common brake-fluid is DOT 3, which is glycol-based. The Department of Transportation (DOT) number indicates the brake-fluid building point. Under pressure, brake fluid gets hot. Slam on the brakes or brake hard continuously and brake fluid temperature skyrockets. DOT 3 means the brake fluid has trouble when there’s 3 percent water in the fluid. This is known as the fluid’s minimum boiling point.

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These are DOT 3/DOT 4 brake fluids. On the left is Valvoline's SynPower High Performance Synthetic Brake Fluid, which is DOT 4 but safe for use with DOT 3. On the right is DOT 3 glycol-based fluid.

Theoretically, brake fluid comes out of the bottle with 0-percent moisture, which means a boiling point no lower than 401 degrees F. Some fluids perform in a range from 460- 500-degrees boiling point. For each percent of moisture absorbed, the fluid’s boiling point drops 50 degrees. That means brake fluid heavily occupied by moisture will boil when you brake hard, creating air pockets and a spongy pedal, commonly known as vapor lock.

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Steel brake lines may look ironclad, but they allow air and moisture into brake fluid. Moisture in brake fluid causes internal corrosion and rust that causes brake line failure. Annual fluid changes protect the lines. Marginal lines must be replaced. Classic Tube can help with custom-bent lines and correct fittings.

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If you fabricate your own brake lines, use double-wall flares as shown. A single-wall flare can fail because line pressure is approximately 1,800 psi.

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This coiled stuff is called anti-chafe, which protects the line from stones and other forms of abuse. You will see it mostly on rear axle lines.

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Brake hoses look like this inside. Despite plenty of layers of reinforcement, moisture will go through brake hoses into the fluid. Brake hoses should be replaced every 4-5 years.

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This is known as bench bleeding, which is bleeding the master cylinder before the lines are installed. Mark Jeffrey of Trans Am Racing recommends this to prevent jamming the piston at bottom dead center. He suggests slowly depressing the pedal until all bubbles disappear, but don't bottom out the pedal, to eliminate all air from the master cylinder.

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Here's the down-on-the-floor method most of us are familiar with. Your objective is zero air by keeping a solid flow of fluid from the master cylinder to the bleeder. If you have a helper, fill the master cylinder and have the helper pump the pedal, then hold the pedal hard. Open the bleeder and observe fluid and air flow. Close the bleeder again and observe air/fluid flow. Do this until all air is gone. Once you do this on all four brakes, do it all over again until the pedal is rock hard and all air is gone. Make sure all dirty fluid is also gone.

DOT 4 brake fluid, which can be mixed with DOT 3, raises the brake fluids minimum boiling point to 446 degrees F. It adds borate esters to the DOT 3 glycol fluid to improve fluid properties and raise the boiling point. DOT 4 is more of a late-model automobile brake fluid, but you can use it in classics as well. You can mix it with existing DOT 3 fluid, or you can use it after a major brake overhaul. We suggest completely flushing and bleeding your system with DOT 4 if you’re going to change over. Although DOT 3 and DOT 4 are compatible, it makes more sense to have one type in the system.

So what are the advantages of DOT 3 and DOT 4 brake fluids? These fluids offer better braking quality than DOT 5 silicone. Properly bled and serviced, DOT 3 and DOT 4 feel better when you apply the brakes. The downside to mineral-based fluids is the moisture they absorb, not to mention the damage they can do to paint if you happen to spill any.

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It's easy to assume synthetic brake fluids are DOT 5 silicone. They are not. Synthetic brake fluids are DOT 4, an improved version of DOT 3, with better properties and a higher boiling point. We recommend DOT 4 whenever you do a complete brake overhaul.

DOT 5 silicone brake fluid can’t be mixed with DOT 3 and DOT 4 fluids. You have to completely flush your system, and ideally, begin with fresh hydraulic components. We suggest fresh calipers, wheel cylinders, and master cylinder because silicone fluid could shock the rubber seals, which have grown accustomed to glycol-based fluids. DOT 5 silicone brake fluid is popular with racers because it has a very high boiling point of 700 degrees F. It’s more stable than glycol-based fluids, and it’s non-hygroscopic, which means it doesn’t absorb moisture, and it won’t damage paint.

The downside to DOT 5 silicone brake fluid is a spongy feel in the brake pedal because the fluid is compressible. It has the tendency to make air bubbles during servicing, which means you have to pour it slowly, and to form sludge when mixed with dirt particles. There’s also zero compatibility with antilock braking systems and some boil issues with silicone brakes additives. DOT 5 silicone brake fluid is not compatible with water, which only matters if there’s any in the system. Water won’t mix with silicone brake fluid, so it will remain separate and boil if fluid temperature hits 212 degrees F.

During normal driving, there’s not much need to worry about moisture levels in the fluid. It’s when we need our brakes badly that moisture in the system can be a problem.

Should you use Braided Lines?

Braided brake hoses look cool to the racing and “look at me!” crowd, but they claos serve a valuable purpose in terms of safety. Unless you’re restoring to concours original, braided brake hoses are a good idea. They offer a greater measure of security because they’re stronger. They also offer a harder pedal because they don’t flex like rubber reinforced hoses.
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How to Bleed Brakes

There are several approaches to brake bleeding. The most conventional involves the humble pickle jar, a vacuum hose, a buddy with a good foot, and a brake-bleeder wrench.

Another approach is one I use at Fly-Ford Racing. When you don’t have a friend to help with the bleeding, I suggest the gravity approach, which is to position jars and hoses at all four brakes, fill the master cylinder, and open all bleeders. The system will purge itself of all air over several hours.

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Trans Am Racing's Power Bleeder enables you to do the job by yourself. It draws all air and fluid from the master cylinder without having to touch a brake pedal. Mark pumps the Power Bleeder until there's plenty of negative pressure (vacuum). Then he cracks the bleeder and watches fluid flow. Once all air and dirty fluid are expended, bleeding is complete.


When you’re alone, you can bleed brakes using another approach as well. Position a jar with clean brake fluid in the bottom at the farthest brake from the master cylinder. Run a vacuum hose from the bleeder to the bottom of the jar. Fill the master cylinder with fluid. Pump the brake pedal at least five times slowly to begin fluid entry. Open the brake pedal and listen for bubbles at the jar. Ideally, you’ll have something to hold the pedal down. Close the bleeder. Do this again and again until no air bubbles are heard. Keep the pedal down and close the bleeder. Move on to the next closest brake and repeat the procedure. Keep the master cylinder serviced with fluid. You may have to repeat this procedure a number of times until you achieve a hard pedal.

Mark Jeffrey of Trans Am Racing demonstrated a power bleeder you can use all by yourself. It creates a vacuum at the brake bleeder. When you fill the master cylinder and open the bleeder, the vacuum draws fluid and air out of the system into a container in seconds.

USED WITH PERMISSION

Marlon Mitchell is the owner and founder of Marlo's Frame & Alignment/Fly-Ford Racing in Chatsworth, California. Be sure to check out all of the specialty services his shop offers the auto enthusiast community.
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