Most folks forget to check brake fluid level, and I can’t blame them; brake fluid level rarely needs topping up; in fact, if it does need topping up, it suggests a brake inspection is required. I’m a mechanic, and I’ll share everything you need to know about brake fluid top-ups right now!
When topping up brake fluid, it’s best to use the brake fluid type specified on the brake reservoir cap. DOT 3, DOT 4, and Dot 5.1 are safe to mix when you’re in a pinch, but DOT 5 must not be mixed with any other fluid type.
In this post, you’ll learn what a low brake fluid level really means, how to add brake fluid, what fluid to use and why it’s important not to mix the fluid types.
Index
- Checking brake fluid
- What a low fluid level really means
- Brake fluid types (mistake to avoid)
- Adding brake fluid
- Sum up
Checking Brake Fluid Level
I recommend checking your brake fluid every time you check your other fluids, oil, coolant, and windshield fluid. Checking brake fluid is the easiest of all to check as the opaque reservoir means we only need to run a visual to access fluid level.
This is true for newer models; as vehicles age, the reservoirs tend to be less opaque and, therefore, harder to read; in those situations, you may need to remove the cap and peer in to check the level.
Anyhow the checking process looks like this:
- Level ground – Park vehicle on level ground to get a more accurate read
- Engine off – Vehicle engine and ignition off
- Locate the reservoir – Located under the hood, directly in front of the driver on the firewall
- Identify the Min & Max marks – Min & Max on the reservoir wall
- Read level – Check fluid against min max marks
What Low Brake Fluid Level Really Means
When checking fluid levels, brake fluid is often forgotten, and you can see why. Brake fluid rarely needs topping up because, unlike other fluids, brake fluid isn’t consumed.
When the brake fluid level is low, it means your brakes require inspection. It could mean you have a brake fluid leak, but you’d know about that pretty quickly, as your brake pedal would sink to the floor and your brake force compromised.
The more usual reason for a low brake fluid level is your brake pads or rear shoes (if applicable) are worn.
Topping up the brake fluid reservoir under these circumstances is actually overfilling the reservoir. When the caliper pistons are pushed back during brake maintenance to allow for the new (thicker) pads, the excess fluid will rush back to the already full reservoir and overflow.
And so, before topping up the fluid, I’d advise checking the brake linings for wear first. If they are worn as we suspect, I’d skip topping up the fluid altogether and go ahead and change the pads out or book a brake service.
Most vehicle wheel designs allow for easy brake pad inspection, and we’ll cover that next.
Brake Pad Inspection
Brake pads are easy to check on most vehicles. The wide-wheel spoke design is great for visibility and inspecting the pad lining thinness and only takes minutes. Most vehicles will have brake calipers on all four wheels.
However, some will only have calipers to the front and will instead have drum brakes to the rear. It is not as easy to inspect drum brakes, and worn shoes (drum brakes) will also cause a low brake fluid level.
That said, front brakes are harder working brakes and are always the more likely brakes to need attention.
The brake pad inspection is as follows:
- Using a phone light works great. Locate the caliper and locate the pad
- Gauge the pad thickness
A new brake pad lining is approximately 10-12mm. Anything below 4mm is considered worn see chart below.
| Lining Condition | MM |
| Ok | 8 – 12 |
| Advisory | 4 – 7 |
| Worn | 2 – 3 |
I’ve covered the inspection and pad replacement process previously in this post, “Brakes making a grinding noise.”
Brake Fluid Types
Brake fluid is brake fluid, right? Wrong, brake fluid is not all the same. And it is a common and sometimes expensive mistake to add whatever brake fluid comes to hand.
Brake fluids are classified by how they perform under extreme braking conditions.
When brakes work hard, they throw off a ton of heat, and your brake fluid can’t help but absorb some of that heat. The heat reduces the performance of the fluid, and so a fluid that is resistant to heat is better, and as such, fluid is classified by its ability to resist heat.
Fluid must meet or exceed the DOT (Department Of Transport) specifications. Two temperatures are looked at, in particular, the Dry and the Wet boiling point.
The dry boiling point is the temperature the fluid boils at with less than 1% moisture by fluid volume (new fluid)
The wet boiling point is the temperature the fluid boils at, with a moisture content of 3.7% by fluid volume (older fluid). Bear in mind glycol brake fluid is hygroscopic; it attracts moisture (about 2 % a year) naturally, and that’s why it’s best to change glycol fluid every two years, three years max.
The higher the boiling point, the better the fluid and is classified as such, see table below.
| Fluid label | Boiling Point | Fluid type |
| DOT3 | Dry 401°F Wet 284°F | Glycol |
| DOT4 | Dry 446°F Wet 311°F | Glycol |
| DOT5.1 | Dry 500°F Wet 356°F | Glycol |
| DOT5 | Dry 500°F Wet 356°F | Silicone |
While you should always use the fluid classification specified on your brake fluid reservoir cap, it is possible to mix glycol-based fluids if you are stuck. Mixing DOT 3, 4, and 5.1 is okay, but ideally, you shouldn’t, as that changes the boiling point. But as said, it won’t hurt the system any if you do mix them.
That is not true for DOT 5; you can not mix DOT 5 with any other fluid but DOT 5. More on why that is below.
So what’s the big deal with moisture in the brake fluid? The problem with moisture in the fluid is twofold. First, it poses a safety concern as when the brakes get hot, your fluid starts to boil, and the moisture content turns to steam, which is compressible.
Compressible fluid means your brake pedal goes further to the floor before the brakes take effect, normal braking is restored when the fluid cools and the steam turns back to water.
The other serious but less imminent risk is internal brake system corrosion; where there’s moisture, there’s a risk of corrosion. As the brake fluid is old, its corrosion inhibitors have long since lost their superpowers. Meaning all the metal and rubber brake system components are under attack.
DOT 5 is different; it’s a silicone-based fluid and is a high-performance fluid that does not absorb moisture and does not need to be changed. It is a more expensive fluid but, in the long run, likely a cheaper option.
DOT 5, unlike all the other DOT classifications, can not be mixed.
Mixing a glycol-based fluid with a silicon-based fluid will require a brake flush at the very least; at worst, brake components may need to be replaced.
Adding Brake Fluid
Adding brake fluid is a simple chore; you already know that you should use the DOT type specified on your reservoir cap, and we’ve covered how it is possible to mix the DOT3, 4, and 5.1 but never DOT5.
Accessing the fluid reservoir on some vehicles can be tricky, and in some instances, you may need to use a long funnel.
DOT 3, 4, and 5.1 brake fluid will remove car paint, so you’ll need to be careful where you spill or splash the fluid. DOT 5 will not remove car paint.
The filling process looks like this:
- Clean – Clean around the reservoir cap before removing
- Spill – Place a cloth around the reservoir opening to catch any spill
- Pour – Add only small amounts of fluid before checking the level
- Clean – Remove any spill immediately
- Refit cap – Close reservoir cap immediately
Be careful not to overfill the reservoir; overfilling may cause the brakes to drag, the fluid volume may change as its temperature changes, and overfilling may cause the expanded fluid to push out the caliper pistons, which causes the brakes to drag.
Sum Up
It’s okay to top up your fluid; the correct fluid type is marked on the brake fluid reservoir cap. DOT3, DOT4, and DOT5.1 should ideally not be mixed; however, they may be mixed if you are in a pinch, DOT5 must not be mixed with any other brake fluid type.
While it is generally okay to add brake fluid, having to do so usually means your brake pads are worn and need replacing.
About the Author
John Cunningham is a Red Seal Qualified automotive technician with over twenty-five years of experience in the field. When he’s not writing about car repair, you’ll find him in his happy place – restoring classic cars.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.