Can I use brake fluid for my hydraulic system? Is brake fluid an easy option?

Plastic bottle of BREMBO Brake Fluid for topping up your system

Brake fluid may be carefully formulated for automotive braking, but how good is it elsewhere in a hydraulic system?

The answer is yes, to a point. However, you must be very careful. It is specialist and certainly isn’t suitable as a substitute for all applications. Where safety is critical, we would never change away from the design-specified fluid anyway. Let’s remember there is a difference between a substitute fluid in the direst emergency and simply swapping fluids because it is easy.

The advantages of brake fluid are convenience: you can buy it at any auto store and don’t have to buy 20 litres. Those might well be big pluses, but the engineering is everything. Is it the right fluid? Once you dig into all the issues around this fluid, you might have second thoughts and operate on conventional mineral oil. But some other specialist hydraulics do make it work with massive success. There are lots of formulations of hydraulic fluids for particular applications that aren’t that far away from glycol brake fluids in composition, so this isn’t a bad question to ask. The key words are “specialist” and “formulations.” Select your fluid carefully. Most important, find out what it is in more general terms.

Brake fluid is probably the most commonly used hydraulic fluid if you look at all the automotive brake and clutch hydraulics in use in the world. Hedges & Company on the web tells us there are 1.4 billion cars in the world. Add in commercials plus motorcycles and you have a massive user base. Keep in mind that Shimano brakes on bicycles use mineral oil. That’s interesting. We have looked previously at what a massive success these miniature hydraulic systems are. If brakes and clutches are a very specialist application of miniature hydraulics for the precise requirement, we think the development of hydraulic fluids for them has ended up quite niche. Brake fluid is generally a specialist hydraulic fluid tailored for this exact use. We wouldn’t use it just because small bottles are available at every auto store. The application and fluid properties are quite precise. Then add in the specific recommendations for looking after your glycol-based brake fluids and that tells us to think carefully before using it in place of mineral oil. We’ll briefly talk about looking after brake fluid as a hydraulic fluid as a heading later.

Who invented brake fluid? 

Hydraulic brakes go right back to pioneering systems by Duesenberg in the USA in 1914 and inventions by Malcolm Loughead, the founder of Lockheed. Hydraulic braking must have been quite slow to be universally implemented, as VW were still using cable brakes on the Beetle up to around 1960.  There is plenty of background on the web.

Why isn’t mineral oil used in automotive braking systems?

brembo brake discs and caliper

There are some very precise and unique requirements for the hydraulic fluid in an automotive braking system. Image courtesy Brembo

 The web tells us that quite early on it became apparent that mineral oil didn’t absorb water. Why does this matter? There was a specific issue with braking systems where under very heavy and hot braking, the water could boil and make the system inoperable.

Water droplets would boil to become a gas and you have a disaster with very spongey brakes. Corrosion in the system due to this water contamination was also cited as an issue. Where do water droplets come from? Water would build up in the hydraulics largely due to condensation during swings in temperature through the day. This same water build up from condensation is an issue in larger mineral oil-based hydraulics, although reservoirs invariably have the volume to retain the water and keep it out of the way in the bottom of the tank. Maybe the miniature reservoir in a braking system makes water ingress more of an issue. How likely is it that heavy braking would vapourise droplets of water? Probably not that often, but if this did occur it makes an emergency action into an even worse problem should the brakes suddenly become spongey. Any Engineer would work around this and provide a solution. And the pioneers of hydraulic braking systems did just that. They found a different fluid. The lower viscosity of the glycol-based fluids would also have been beneficial with unrestricted flows in small bore pipework. Higher viscosity fluids are noted as making brakes less responsive, so this must also have driven the early move to glycol. At low temperatures, mineral oil would become quite viscous without a recipe of additives. As always, you should never say never. Citroen used and still use their mineral oil based LHM fluid with success, although we understand that it is regularly changed to dodge issues with water droplets possibly boiling and causing sponginess. However, it is rated highly on low corrosion and good seal wear. There are additives to maintain low viscosity at low temperatures. Note that Rolls Royce also used the Citroen system in the past.

What brake fluids are used today?

There are plenty of web resources on the various DOT designations for brake fluid. (DOT stands for the US Department of Transport.) We have discussed briefly water glycol fluids, mineral oil-based fluids and we will later mention silicone oil-based fluids.  This US guide is comprehensive and explains all.  

What does my auto shop do when it “tests” my brake fluid?

Hydraulics people tend to see mineral oil as quite a forgiving fluid for long term use. Yes, cleanliness is everything, but that is true of any hydraulic fluid.

For water glycol-based fluids like brake fluid, ongoing checking of fluid becomes important if fluid readily absorbs water from the air. Your garage or auto shop will commonly test the water content and may even swap the fluid out regularly even without testing. Our BMW was two years. Once the water content becomes too high, the issue of fluid boiling at high temperatures rears its head. The fluid is no longer effective for the job.

Another issue with water glycols is the corrosion inhibitor ceasing to remain effective and the fluid becoming acidic. This increased corrosion from the acidity is a risk to the hydraulic system components. An auto shop can test the acidity but maybe it tends to be swapped out anyway long before this problem rears its head. All we know is that over many years of supplying our Micropac pumps, professional users of water glycol based offshore hydraulic control fluids have told us that they prefer pumps in very low corrosion 316 stainless steel compared with aluminium alloy system components. Engineers are quite paranoid about how destructive this corrosion is due to acidic water glycols. Beware. There is certainly some issue here with water glycol-based fluids and corrosion over longer periods. That is bad news if your braking system components are alloy. The same is also true with anti-freeze in an automotive cooling system where over extended periods, it can turn acidic and start to corrode metallic components. Neglecting your antifreeze in an engine is quite destructive. This all highlights a real issue with water glycol-based fluids. These fluids need maintenance.

Why is silicone brake fluid used? 

Silicone brake fluid is a specialist “DOT 5” fluid used on military vehicle, classic or show cars and niche applications, so we are told. Note that silicone oil used as a brake fluid does not absorb water unlike glycol-based fluids. It also has a high boiling point. There must still be the issue of water droplets turning to gas at high temperatures, but maybe this is a low risk nowadays. Tell us the answer! In our hydraulics world, silicone oil is used in specialist hydraulic applications like calibration and damping.

Whereas water glycol-based fluids will absorb water, silicone fluids carry water as droplets. That goes back to the big benefit of glycol-based fluids all those years ago. Silicone fluid has higher viscosity than the glycol media which in itself could potentially provide less responsive brakes as you will have increased pressure drops through the system. We are also told that silicone oil is more compressible than glycol-based fluids, translating to more foot pedal action required to get the same effect.  The increased compressibility probably relates to silicone picking up more dissolved air. Remember that even everyday mineral oil has around 10% of dissolved air in it and that some critical applications do require that you pull the air out using a vacuum pump and chamber. If silicone picks up similar amounts of air, you are not going to get such a positive action if water glycol has less air in it.  Silicone fluid will provide additional corrosion resistance compared with the glycol-based fluids which ultimately do allow system corrosion once they have absorbed a certain amount of water. This very long service life can be a big plus, lifting the need to regularly change or monitor the condition of glycol based automotive fluids. Classic car folk particularly like silicone fluid because it is kind to paintwork compared with glycol fluids.

So, in answer to the question, silicone brake fluid has distinct benefits for some users, but some disadvantages compared with glycol-based fluids. Again, feel free to comment on silicones. Merlin Motorsports have a good write-up.

Would I use glycol brake fluid on a hydraulic system with a motor driven pump?

We would say no. Mineral oil is a very good lubricant for elastomeric seals. Clearly, an automotive braking system uses rubber seals, but they will be assembled with grease and relatively speaking, the number of cycles isn’t massive compared with a powered system. Again, the relatively slow speeds of operation of the brake system seals and the limited number of cycles over a lifetime are one factor in allowing the use of water glycols. Move to lots of cycles at higher speed and you either change to mineral oil or a glycol-based fluid with “slippery” additives in it or go the route water hydraulics people like us go and use capped seals in materials like filled PTFE or UHMWP. Using elastomeric seals on a hydraulic fluid with poor “lubricity” can be very problematic. Beware.

What about other water glycol-based fluids for hydraulics?

A steel mill producing red hot bars and hydraulics running on HOUGHTO SAFE 620 fluid

A steel mill can’t risk inflammable hydraulic fluid and would use Houghtosafe 620 or similar in place of mineral oil

 If you look slightly away from glycol-based brake fluids for your hydraulics, you are into quite a big sector of the hydraulic fluids market. We have previously asked the question “can I use tap water for my hydraulics?” It is a great white paper well worth a read. Once again, you are picking the properties that you need in a hydraulic fluid and formulating a recipe to meet that requirement. Maybe one “simple” application is the offshore control fluid mentioned previously.

McDermid HW series fluids are used in offshore control systems and Blow Out preventers. The formula allows use at high temperatures, over long periods and with limited environmental damage. The fluid is low viscosity for small hoses over long distances. Other water glycols are used in marine applications where you need a fluid that is less damaging environmentally in case of a spill and maybe fire resistant for defence applications. If the fluid is used with hydraulic motors and valves, it will be more viscous and have anti wear additives to allow it to operate on mainstream hydraulic components. Our article concluded that tap water was an exotic hydraulic fluid for lots of reasons. Bacterial contamination has not been mentioned yet, but that is a real issue on water and requires biocides. Shell has a good history and interesting guide on water glycols. For sectors such as steel making, defence and mining, fire risk from burning hydraulic oil is a major issue. One manufacturer who offers specialist hydraulic fluids spanning a massive range of industries and requirements is Quaker Houghton. We have used Houghtosafe 620 fire resistant fluid on our equipment. They are good people.

The answer to the question on using water glycol fluids other than brake fluid is very much yes, do consider it.

You need to know what properties you need but inevitably will end up with a specialist cocktail with a trade name unless you can steer within a standard like “NATO OX40” and have a range of fluids on offer. Maybe a sensible Engineer would start with not adopting a fluid that can only be purchased in 20L drums or even worse, 207 litres. 5L is a more useful size for many users. Finding that a customer cannot buy a top up for less than hundreds of dollars has implications.

One final comment on using these alternative fluids is that the field is massively crowded and complicated. Our tip is to try to select something that has a generic classification like “HFC” or even “complies with a NATO or US MILSTD.” A long running problem in our business has been users who adopt a very specialist proprietary fluid with a trade name, and we just cannot find out what it is to select the right materials and seals.

Is a water glycol brake fluid better for the environment than mineral oil?

Oil Spill Containment Methods | response.restoration.noaa.gov

Oil spills and the contamination of any water is an environmental disaster.

Our original customer in 1982 was a firm who wanted to use water for a trench shoring system. As we have noted, that is a fine idea except that water invariably needs a biocide for extended use and immediately you are moving towards a formulation or cocktail to provide your fluid. Weaning the world off mineral oil is most definitely the future. Just be aware that although water-based fluids like water glycols offer a solution, use and disposal of them isn’t without issues.  Water Glycol “Antifreeze” is quite toxic, so care needs to be taken. There is an issue here with diethylene glycol, as outlined on the web. Beware.

Which hand pumps will work on brake fluid? What is special about a Micropac pump?

There are a few issues here that our Micropac pumps address. The starting point is that you need the right SEALS. Our kit has always been designed to allow us to offer different grades of elastomer easily and cheaply. If the brake fluid is low viscosity, you would be wise to use a pump like our MW with a poppet outlet non return valve and a soft seat release valve. Designs that use a ball peened into a metal seat aren’t our choice for low viscosity fluids. Low corrosion materials like stainless steel 316 would be our first choice. Use anodised alloy if you are careful with the state of the fluid. Finally, if you ask for the pump to be tested on brake fluid, that’s what we will do. The big manufacturers are unlikely to offer that service. Look at our MW series, MP carry arounds, the MB low pressure high-capacity units or the MT-6 cartridge pump for low viscosities. Talk to us about your application.

We are the Multifluid pump specialists.

Our pumps were designed over 40 years ago to allow you to use the correct materials and seal compounds for your fluid. You configure precisely what you need. After all these decades, we are the experts. Contact us.

We hope you found this paper useful. We are not chemists or physicists, so feel free to comment and help us understand better.