Ask Away! with Jeff Smith: Which DOT Brake Fluid is Best for My 1962 Chevy Impala Brake Upgrade?

My brother and I are restoring my dad’s 1962 327 Impala. We’re going to upgrade the suspension and brakes and the question came up as to what brake fluid we should use. While the traditional DOT 3 brake fluid is the cheapest, we thought maybe we should upgrade to a more high performance fluid. The old original wheel cylinders for the rear drum brakes were all so badly rusted that we’re going to replace them but keep the rear drums.

My idea was a high performance brake fluid like a DOT 4 would work better but we thought we’d look into this first. 

B.F.

As cars become more sophisticated and the performance of disc brake systems has elevated, so has the demand for brake fluid that will match these higher performance and temperature levels. New performance cars now come with 6 piston calipers and monster front brakes designed to survive on a road course. But with higher performance fluids comes other limitations.

It sounds like your car will probably just be a weekend cruiser, so the short answer is, since you won’t be pushing the brakes hard or creating a ton of heat, the stock replacement DOT 3 brake fluid is your best choice.

But let’s look at all the different fluids and why this plan has merit.

The Differences Between Brake Fluid DOT Specs

As with most questions I get around high performance choices for automotive systems, it’s best to first determine how the vehicle will be used and then base the answer on that application. With brake fluid, what differentiates the various performance levels is the fluid’s boiling point. The U.S. Department of Transportation (DOT) has established minimum performance levels for the different grades of fluid. We’ve included a chart that lists these minimum boiling points.

Brake Fluid Boiling Points

With the exception of silicon-based DOT 5 fluid, the remaining DOT 3, 4, and 5.1 fluids all use a glycol-ether material as the base fluid. This fluid is also hygroscopic, which means when exposed to the atmosphere the fluid will absorb water right out of the air. This is why you should always keep the lid to any brake fluid container tightly sealed until it is ready to use.

The wet temperature standard set by the DOT is when the fluid absorbs a minimum of 3.7 percent water. This isn’t very much but as you can see from the chart this small amount of water has a dramatic effect on boiling point. The boiling point is important in situations where the brakes generate sufficient heat to exceed the fluid’s boiling point in the brake calipers. When this happens, the water in the fluid boils first and creates bubbles that compress.

This creates the situation where under hard use, the brake pedal travels right to the floor because the air has compressed and the hydraulic circuit no longer functions properly. Unfortunately, this occurs at the precise moment when you need the brakes the most.

So you can see why boiling point of fluid is important.

This might point to using a higher rated fluid like a DOT 4 or even a DOT 5 or 5.1. As the DOT standards increase, other chemicals are added to increase the boiling point of the fluid in both wet and dry situations. However, as the fluid evolves from DOT 3 to DOT 4 to DOT 5.1 (we’ll deal with DOT 5 separately), this also increases the fluid’s sensitivity to water. Essentially, the higher the rated fluid, the more quickly it will absorb water.

Factory recommendations for DOT 3 are to change the fluid every 2 to 3 years. Of course, we all know that very few people change their brake fluid that often. With the DOT 4 and DOT 5.1, the factory recommends changing the fluid every 1 to 2 years. This gives you an idea of how much more sensitive DOT 5.1 or DOT 4 is to water. This is why for your early car we’d recommend staying with the DOT 3 fluid as it will be more stable over a longer period of time. One way to tell if the fluid needs changing is if it becomes very dark and cloudy. That usually indicates the fluid should be flushed and replaced. 

Special Considerations for DOT 5 Brake Fluid

You may have noticed from the chart that the DOT 5 brake fluid appears to offer much higher performance. This is a silicon-based fluid that does not absorb water nearly as easily, so it tends to have a much higher boiling point. The downside to this fluid is that it tends to generate air internally when heated which creates a less firm pedal when exposed to performance conditions.

However, it may work well for older or antique cars that are not driven very much and never overheat the brakes since the DOT 5 fluid tends to be more stable over time. If you are interested, you should look into the positive and negative details of using a DOT 5.

This DOT 5 should never be used in cars with ABS or anti-lock brakes and DOT 5 should never be mixed with any other fluid.

So, Which Brake Fluid Should You Use?

For newer cars that require DOT 4 or a DOT 5.1 brake fluid, you should never try to use a lower rated fluid like a DOT 3 unless it’s an emergency. However, older cars can benefit from the higher boiling point of a DOT 5.1 in a performance situation like autocross or track day runs where the brakes will be used to their fullest extent.

The entire automotive industry is changing and has become more sophisticated. The old days of one brake fluid for all cars has now evolved into many more requirements that make decisions for simple things like brake fluid now more complex.

Hopefully this explanation clears up some of the possible confusion.

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