When brake pads are worn to their limit, chances are the driver never noticed. Since pedal travel doesn’t change much as disc brake pads wear and all of today’s drum brakes have self-adjusters, even brakes that are just about worn out will still feel normal and stop the car effectively. This explains the surprised look on many drivers’ faces when technicians tell them they need to spend money on brake repairs. However, most drivers will notice what the pedal feels like, even if it changes gradually. Assuming the booster works properly, everything the driver feels at the pedal is transmitted through the hydraulic system.

Hydraulic fluid in a brake system feels and in some ways behaves like a solid mechanical part, a mechanical link between the master cylinder and slave cylinder. When the driver pushes the pedal, he feels the resistance to movement all the way at the other end of the line as the pads squeeze the rotor. If the rotor is warped, he feels the pads pushing back as the high spot moves between them. The feeling would be exactly the same if the pedals were connected directly to the pads through a straight metal rod. But if something happens to make the fluid behave differently, the pedals get a spongy feeling, like there are rubber bushings on the rod ends.

Basically there are only two things that change the way hydraulic fluid behaves in a brake system: leakage or fluid contamination. The effect of a leak, either external or internal, is fairly obvious even to the least knowledgeable driver, long before he sees the red warning light on the dashboard. Contamination is a little more mysterious, and it can cause some unusual problems that even experienced technicians won’t recognize right away. Almost every hydraulic brake system suffers fluid contamination at some point, but it can be prevented. That means there’s an opportunity for you to sell a valuable service to your customers, but we all know that sometimes that’s like pushing on a rope. To make an effective sales pitch, it will help if you know a little more about it.

What does it need to do?

All fluids are basically non-compressible, meaning that they won’t compress into a smaller volume when pressure is applied. But there are limits to the non-compressibility of any fluid. Water is the most non-compressible fluid, and hydraulic systems have been successfully designed and tested using water as the working fluid. But for a variety of reasons, such systems are impractical for most applications. In a brake system, fluid pressure is multiplied by the master cylinder and can reach more than 1,000 psi in the lines. As the pressure builds, the various cups and seals in the system are shaped so they expand slightly, increasing their pressure against the bores to form a seal that holds the pressure.

Like any other hydraulic fluid, brake fluid must be non-compressible at the expected pressures to transmit force from one end of the system to the other, and it must lubricate the pistons and rubber parts as they move through their bores. But unlike any other hydraulic fluid, it has one big liability: There is almost no fluid flow. Even though an antilock brake system (ABS) circulates fluid through part of the system, brake fluid simply moves slightly back and forth for most of its life. There is no opportunity to circulate the fluid through any kind of conditioning device like a heat exchanger or a filter. Because it can’t be filtered, brake fluid ideally should hold any contaminants in suspension to prevent corrosion or clogging of small passages.

Like all other industrial and automotive fluids, hydraulic fluid is tailor-made for the job at hand. One of the critical factors engineers need to consider when selecting a hydraulic fluid is the expected working temperature range, because fluid viscosity changes with temperature. When the fluid is expected to do work as opposed to just lubricate (like motor oil), viscosity must remain constant for the hydraulic system to perform consistently.

In a power steering system, the fluid is at ambient temperature at start-up and then absorbs heat from the pump and engine. Temperatures can vary from ambient to 300° F, but after circulating for several minutes, the fluid temperature will be roughly the same throughout the entire system. Brake fluid must work in a wider temperature range, and because the fluid doesn’t circulate, there can be quite a large difference in fluid temperature from one end of the system to the other. Some of the fluid warms up as underhood temperature rises and fluid in the calipers and wheel cylinders absorbs heat from the brakes, but the fluid in the lines never gets much above ambient temperature. Brake fluid viscosity must remain constant over as much as a 500° F temperature difference within the system. Since part of the system might never warm up above ambient temperature, that viscosity must be very low so it can still flow when cold. This is especially important with ABS, because some of the fluid in that system does circulate.

But low-viscosity fluids generally have a lower boiling point. All fluids vaporize when they boil, and vapor can be compressed – the exact opposite of what you want in a hydraulic system. In most other hydraulic systems, the fluid is always pressurized, which raises the boiling point and stabilizes viscosity, but fluid in a brake system is only pressurized when the driver uses the brakes. (Systems have been designed that keep the brake fluid under constant pressure, but they’re very complex and not suited for automotive use.) Fortunately there are a few low-viscosity fluids with a high boiling point, and additives can be used to raise the boiling point even higher.

What’s in there?

The three basic materials used to make hydraulic fluid are mineral oil, glycol-ether and silicone. DOT 3 and DOT 4 brake fluids are made with glycol, which has a stable viscosity over a wide temperature range and flows easily at low temperatures. It also is non-compressible at even the highest pressures generated in automotive brake systems. Glycol will attack some types of rubber, causing them to swell and deteriorate, but compatible materials are available and not expensive. It’s also a good lubricant, and an appropriate additive package can raise its boiling point to more than 450° F. The main problem with glycol is that it will absorb and mix readily with water. This makes it ideal for use as antifreeze in a cooling system, but in a hydraulic system it’s a "good news/bad news" situation. The good news is that the water mixes evenly with the fluid and remains in suspension, slowing its corrosive effects on steel, iron and aluminum parts. The bad news is that it absorbs water quickly, even minute amounts of moisture that seep into a brake system through seals and line fittings. If the cap is left off the reservoir overnight anywhere except in a desert, the fluid can absorb enough water from the air to cause real problems. We’ll describe those problems later.

Mineral-oil-based hydraulic fluid is completely non-hygroscopic, meaning it won’t absorb or mix with water. While water won’t change its boiling point, any water that does get into the system tends to collect in small droplets or slugs that will boil at a lower temperature than the oil. The small slugs also will settle and corrode metal parts more deeply than if it were mixed with the fluid. Oil by itself has its limits as a hydraulic fluid because its viscosity changes with temperature. It also will attack the seals and cups made for glycol systems, so once again, the materials must be carefully selected for the application.

With all these disadvantages, you’d think oil-based brake fluid would be a big mistake, and you’d be right. We aren’t aware of any automotive braking systems that use mineral-oil-based brake fluid today, but it is sometimes used for heavy-duty wet-clutch and wet-brake applications. If the system is designed specifically for the use of mineral oil fluids – and if it remains water-free – oil-based hydraulic fluid works just as well as glycol. But the two different fluids can’t be mixed. No doubt you’ve seen warnings about adding mineral-oil brake fluid to a system not designed to use it. That’s mostly to prevent deterioration of the cups and seals.

DOT 5 silicone brake fluid also is non-hygroscopic, can handle very high temperatures without boiling and retains its viscosity over the widest temperature range of any hydraulic fluid. It also won’t attack rubber brake system parts. Barring contamination, vehicles that leave the factory with silicone brake fluid never need to have the fluid changed and almost never have hydraulic system leaks. In fact, it’s so non-reactive that it won’t ruin the paint if accidentally spilled on the bodywork, which may be the reason it’s used in some motorcycles with the fluid reservoir on the handlebars.

Being non-hygroscopic and completely inert makes it a good brake fluid for vehicles that are used infrequently or stored for long periods, like a show car or an old reserve fire truck that sits in the back bay most of the year without moving. The U.S. military prefers silicone brake fluid as a factory-fill for just that reason. But silicone fluid is not ideal for all applications. One problem is that its non-compressibility limit is lower than other fluids, which might occasionally produce a spongy pedal feel in a panic stop. A bigger problem is that it tends to foam when agitated or passed through small orifices. That’s why it can’t be used in vehicles with ABS, because it would foam and become dangerously compressible when circulated through the pump and small valve openings. Silicone also is less effective as a lubricant in the interference-fit environment of a hydraulic system, producing a bit of striction upon the first pedal application after sitting for a long time. And like a system that uses mineral oil, it absolutely cannot be mixed with any other fluid. Any water in the system will cause all the same problems as water in an oil-based fluid. Still, as long as there is no contamination and it’s not used in ABS brakes, silicone fluid is a good choice in some brake system applications.

Failure modes

There are two basic failure modes for brake fluid: It can boil, and it can cease to provide adequate lubrication and corrosion protection. Both are the result of contamination, usually water and/or petroleum products. We noted earlier that glycol brake fluid is hygroscopic; it absorbs water easily and holds it in suspension just like antifreeze does. We’ve all heard or read somewhere that water reduces the boiling point of brake fluid, and the federal government has set standards specifying both the wet and dry boiling point of brake fluid.

When it’s new, DOT 3 brake fluid must be able to reach no less than 401° F before boiling at atmospheric pressure. The wet boiling point is specified as 284° F. That doesn’t seem very high, but in most brake systems, the fluid in the caliper won’t reach that temperature unless the brakes are abused, and usually the friction material would reach its designed temperature limit, too. But in DOT 3, it only takes a 4-percent solution to reduce the boiling point down to the federal limit. In most climates, moisture seeps into the brake system continuously through the various seals and even right through the microscopic pores of the flexible brake lines. The seepage can accelerate as a vehicle ages, and there’s almost no limit to how much water the fluid can absorb (what’s the typical antifreeze mixture?). By the time fluid has been in the system for three years, it can easily reach the wet boiling point limit. The real problem with all this is that it happens gradually. While the pedal may feel a little "soft" in normal driving, most drivers won’t discover the reduced braking ability until the worst possible moment.

DOT 4 brake fluid is also glycol-based but it has a different additive package, raising the dry boiling point to 446° F and the wet boiling point to 311° F. The higher boiling points are appropriate for high-performance cars and motorcycles and for vehicles used for towing. There is also a synthetic DOT 4 from Valvoline with wet/dry boiling points of 513° F/340° F. As you might expect, it’s more expensive, but it’s a good alternative to DOT 5 (500° F/356° F).

DOT 5 is commonly used in racing, but preparing a brake system for the track is not as simple as flushing all the glycol fluid out. Silicone and glycol fluids won’t mix, and if even tiny amounts of the old fluid remain in the caliper, it will form small slugs that will probably boil and cause the brakes to fade before finishing the third lap. The only way to convert a system to DOT 5 is to remove and rebuild all the hydraulic parts using DOT 5 as assembly lube. DOT 5 fluid contains a blue dye to make it easier to avoid accidentally adding it to the wrong reservoir. It’s surprising how often this happens in the real world.

In a conversation with Richard Baumgart of Valvoline’s brake fluid division, we learned that fluid contamination from grease and oil is far more common than you might think. Simply using a petroleum-based cleaning fluid to wash a caliper or wheel cylinder might be enough to get things started. Petroleum contamination causes swelling of the internal cups and seals, creating a spongy pedal feeling in as little as a few days. If that happens, simply flushing the brake fluid won’t make much difference because the damage to the rubber parts is permanent.

Brake fluid is a maintenance item

So now that you know more than you ever wanted to know about brake fluid, what do you say to both professional customers and DIYers that will convince them they need to change it? You could simply point out that many manufacturers recommend fluid flushing every so-many years or miles. Some have just recently added this to their maintenance schedule; others have considered it a maintenance item for years.

If your customers need more convincing, there are some products available specifically designed to read the level of contamination in brake fluid. Measuring the moisture content is a good indication of the fluid’s ability to resist boiling, but that’s not the only reason to change brake fluid. The corrosion inhibitors in glycol lose their effectiveness in brake fluid just as in a cooling system, usually long after the fluid discolors but often before the moisture content is above the allowable limit.

There is a product made by Phoenix Systems intended as a sales tool to provide graphic proof that the driver’s brake fluid is basically "worn out" even before moisture levels become critical. We tried their product in several different fluid reservoirs and learned that it works as advertised to measure what they call "alpha" contamination – the breakdown of the corrosion inhibitors used in all glycol brake fluids. But it won’t detect petroleum products.

The only way to find out if there’s oil or silicone in the brake fluid is to collect it in a clear, clean container and examine it. Since they won’t mix, they’ll form "bubbles" in the fluid that can be easily seen. Replacing every cylinder and caliper in the system would be a tough sell, but if one of your professional customers knows that oil was mistakenly added to the brake fluid reservoir, it’s the only way to guarantee his work.

Whatever tools that are used, it’s important that both professionals and DIYers understand the need for brake fluid maintenance. It not only reduces the possibility of hydraulic leaks as a car ages, it also reduces the possibility of sudden failure just because the brakes are being used unusually hard. Pads and shoes that are overheated will still do something to slow the car, but when the pedal goes right to the floor because the fluid has leaked or vaporized, all that’s left is that awful, helpless feeling.

Photo courtesy of Phoenix Systems.