You’ve noticed that the power steering fluid levels are running low, and you want to top it off, but the only lubricant you have is brake fluid. Both systems are hydraulic, so what’s the harm in using the brake fluid instead?
The power steering and brake oil are both hydraulic fluids, but their chemical composition is very different. Because of that, you cannot use the two fluids interchangeably, as they’ll cause damage to the power steering pump, master cylinder, and other expensive components.
We’ll discuss the differences between power steering and brake fluid and why it’s dangerous to use them interchangeably, as well as possible alternatives in a case of emergency.
Brake fluid is a lubricant that transfers force into pressure and increases the braking power. The system works on the hydraulic system, which is based on the non-compressible qualities of liquid.
The production of brake fluid is kept to high standards. There are several governing bodies that determine the requirements, such as the International Standards Organisation (ISO standard), and the Society of Automotive Engineers (SAE standard). The most popular standard is made by the Federal Motor Vehicle Safety Standards – DOT standard.
The following table displays the basic characteristics of brake fluids based on the DOT standard.
|
Standard |
Dry Boiling Point | Wet Boiling Point | Viscosity at -40° C | Viscosity at 100° C | Primary Constituent |
|
DOT 2 |
190° C | 140° C | / | / | Castor oil or alcohol |
| DOT 3 | 205° C | 140° C | ≤ 1500 mm2/s | ≥ 1.5 mm2/s |
Glycol ether |
|
DOT 4 |
230° C | 155° C | ≤ 1800 mm2/s | ≥ 1.5 mm2/s | Glycol ether or borate ester |
|
DOT 4+ |
230° C | 155° C | ≤ 750 mm2/s | ≥ 1.5 mm2/s | Glycol ether or borate ester |
| DOT 5 | 260° C | 180° C | ≤ 900 mm2/s | ≥ 1.5 mm2/s |
Silicone |
| DOT 5.1 | 260° C | 180° C | ≤ 900 mm2/s | ≥ 1.5 mm2/s |
Glycol ether or borate ester |
The viscosity plays an important part in the effectiveness of the brake fluid. The fluid has to maintain the same viscosity under high heat and extreme cold to function properly, especially in systems with ABS, ESP, and traction control which use micro-valves and rapid activation. The DOT 4+, DOT 5, and DOT 5.1 are the fluids with the lowest viscosity
The boiling point matters as brake fluid can reach high temperatures in the system, especially in sections close to brake calipers. Vaporization is a serious concern as gas is compressible, and therefore negates the hydraulic effect. You might have experienced this if an air bubble got caught in the brake system.
Brake fluid not only has to be non-corrosive, but it also has to protect the calipers, wheel cylinders, brake lines, ABS module, and other components from corrosion. To achieve this effect, additives and corrosion inhibitors are added to the fluid compound. An attempt has been made to move away from glycol-based mixture and use noncorrosive silicone oils, but the drawbacks, primarily the incompatibility with the ABS system have limited its spread.
To summarize, brake fluid is made out of three primary components: glycol ether or borate ester, lubricating synthetic materials, and anti-corrosive additives.
Power steering fluid also acts as a hydraulic liquid. Instead of affecting the brakes, it boosts the steering and allows you to turn the steering wheel effortlessly. Old vehicles did not have a power steering pump, and a significant amount of force was required to turn the steering wheel when the vehicle is stationary.
The basis for the power steering fluid is either mineral oil, silicone, or a synthetic base. This implies that the power steering fluid has more in common with automatic transmission fluid than brake fluid.
The fundamental difference in chemical composition is the reason why brake and power steering fluids are not interchangeable.
Even though the effect is inhibited through additives glycol ether or borate ester used for brake fluid is still a corrosive substance. This would cause the rubber seals to wear out as they are not capable of withstanding the chemical compound.
The second, more serious problem is the lack of lubrication. Power steering fluid has stronger lubricating properties than brake fluid. Using the brake fluid as a replacement will result in insufficient lubrication and will quickly wear out the power steering pump.
The automatic transmission fluid is often very similar in composition to the power steering fluid, to the point where some manufacturers advise you to use it for both applications.
Because vehicles use significantly less power steering fluid than transmission fluid, manufacturers will create different packaging and advertise the fluid at a different price. However, to know whether you’re getting tricked or not, you have to check the owner’s manual and find out exactly the type of fluid your power steering needs.
Ultimately, the price difference is not significant and whether you’re buying your oils on Amazon, in your local store, or at the gas station, they’ll have a wide selection of engine, transmission, brake, and power steering oil, so you should buy a reserve and keep it in the trunk of your car.
The extent of the damage and risks of using the wrong fluid depends primarily on the mileage you’ve put through your vehicle after topping up.
If you’ve poured brake fluid into the power steering reservoir by mistake and realized it before even starting the engine, then your power steering pump is at no risk of damage. However, driving for an extensive period of time may cause irreversible damage to the power steering pump, which is not the cheapest part to replace.
Whatever the case, the course of action is the same – flush the fluid out of the system completely, and replace it with proper power steering fluid.
There is no real substitute for power steering fluid unless it accepts automatic transmission oil. If that’s the case, then you don’t have to buy the power steering fluid, and can use the exact oil type used for your transmission.
No, because DOT 3 is glycol-based and will eat away at rubber seals while causing damage to the power steering pump due to a lack of lubrication.
In theory, if the oil compounds match, you might be able to, but I would highly advise against it. There’s no reason not to buy dedicated power steering fluid for your vehicle, especially when the failures you’re risking far exceed the cost of fluid.
The primary characteristics of brake fluid are viscosity and heat resistance, but not lubrication. On the other hand, a power steering pump requires a lot of lubrication that brake fluid is not capable of delivering. Over time, the lack of lubrication will wear out the pump and cause a failure.
With the engine cold, raise your vehicle on jack stands so the front wheels are in the air. Open the power steering reservoir and add fluid if necessary, but leave the cap open. Turn the steering wheel fully to the left and right 10-20 times. This will force air out through the reservoir, so you’ll have to add more fluid.
Once the air bubbles stop appearing, close the cap and start the engine. Turn the steering a few more times and check the reservoir level again. Top up if necessary and repeat the process until the fluid level settles.
To bleed power steering fluid, you have to raise the vehicle and remove the skid plate to access the low-pressure hose. Place a pan underneath the hose and disconnect it and that will cause all the hydraulic liquid to bleed out.
Even though the power steering system runs on a hydraulic system, you can’t just use any hydraulic fluid in it. The chemical composition varies greatly between fluid types, and you need to use the right one, otherwise, you’ll damage the power steering pump.
No matter the reason, you should never use brake fluid as a replacement for power steering fluid. There’s a high risk of damage to the power steering pump and given the widespread availability of power steering fluid, you should always choose to go with the proper fluid for your vehicle.
Read these articles for more insights about brake fluids:
How Long Does It Take To Bleed Brakes
Why Does My Brake Pedal Go To The Floor?
Why Are My Brake Pedals Hard And Brakes Locking Up?
