How To Choose And Bleed Brake Fluid: Brake performance depends on it

Mustang Monthly MagazineMustang Monthly

July 2006

By: Jim Smart
Photos by: Jim Smart

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 Loughead, one of the original founders of aerospace giant 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.

Fords didn't get hydraulic brakes until the '30s due to cost considerations. 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.

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.

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.

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.

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.

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.

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 boiling 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.

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.

DOT 4 brake fluid, which can be mixed with DOT 3, raises the brake fluid's 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.

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 brake-fluid 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.

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 we learned from Marlon Mitchell of FlyFord Racing. When you don't have a friend to help with the bleeding, Marlon suggests the gravity approach, which is to position jars and hoses at all four brakes, fill the master cylinder, and open all bleeders. Although we haven't tried this ourselves, he says the system will purge itself of all air over several hours.

When you're alone, you can bleed brakes using an approach we've done many times with success. 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 bleeder. Slowly press 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.

You can use a power bleeder 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.

Brake Lines

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.

Brake Line With Fitting

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.

Braek Line Gravel Guard

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

Inside of a brake hose

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.

Master Cylinder Bench Bleeding

This is known as bench bleeding, which is bleeding the master cylinder before the lines are installed. Slowly depress the pedal until all bubbles disappear, but don't bottom out the pedal, to eliminate all air from the master cylinder.

Manual Brake Bleeding

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 with the pedal fully depressed. Have your helper pump the pedal, then hold hard. Open 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.

Braided Brake Hoses

SHOULD YOU USE BRAIDED LINES? Braided brake hoses look cool to the racing and "look at me!" crowd, but they also 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 hard pedal because they don't flex like rubber reinforced hoses.

 
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