The Basics of Brake Fluids and Brake Bleeding
You’re driving down the road and come to a stoplight. As you depress the brake pedal, you notice it’s a bit spongy and it sinks a bit farther than it did in the past. It might be time to bleed your brakes.
Braking systems can get air into them over time. This can happen for a number of reasons, but air, water, and heat can team up to cause mayhem in the system. Sometimes that takes years, sometimes it’s a lot faster.
Any brake system may have a trapped air bubble, or the fluid level in the master cylinder may be low allowing air in, like an old car I bought once with a master cylinder that was almost dry! Also, brand new brake fluid may have absorbed water due to a poor seal on the container – that happens, ask me how I know. Heated fluid converts water to steam, and both steam and regular old air are compressible, adding a ‘spring’ in your brake system. Air can also get into your brake system when you’re changing or upgrading brake components.
So, you need to bleed your brakes, If you’re a car guy or gal, you have probably already done this job a time or two. Many people have their ‘go to’ method for brake bleeding; we just felt an overall refresher on the job might give you some ideas for the next time.
Your first task is a trip to the auto parts store to grab some fluid. Other than fluid, at a minimum, you’ll need a short bit of clear tubing, and some sort of container to catch the stuff you bleed out. More on tools later.
There are a number of different types of fluids and you should pick the most appropriate for your car. Each fluid type has a number on the bottle – DOT 3, DOT 4, DOT 5, or DOT 5.1 are the most common. If you’ve got an older vehicle it probably came from the factory with DOT 3, and that is generally a good choice, IF your vehicle came with that from the factory. If your vehicle was built new with some other fluid (see the owner’s manual, repair manual, or do some research online), that will be your first choice. If you have built a non-stock vehicle to go AutoCrossing or to tackle other sorts of performance driving, the brake caliper manufacturer will have a recommended fluid type.
The different types of fluids all do the same job, so why so many? Generally, as the DOT number increases, so does the temperature capability of the fluid. This is not a big deal for a street car but it’s important for any kind of racing. Boiling brake fluid is more than just annoying, it can be dangerous or even deadly. You don’t want boiling brake fluid.
A couple words of caution about brake fluid. First, many types of brake fluid will strip the paint from your car, so be careful handling them. Second, some types of fluid can technically be mixed with others, though I recommend you don’t mix fluid types. If you’re switching from DOT 3 to DOT 4, for example, drain all the DOT 3 and blow out the lines before you add the DOT 4, or any other DOT for that matter. In a pinch, if both fluids are natural (non-synthetic) you can use any of them to ‘top off,” but I wouldn’t unless I was caught out on the road with no tools or time to do a proper job.
DOT 3 is the most common fluid. It’s been proven by millions of vehicles for decades and there isn’t anything wrong with it, except that it can absorb moisture in the air by about 2-percent of its volume every year it’s in your car. If you’re not sure if yours has ever been changed, it’s a good idea to change it now. You know why.
DOT 4 is interchangeable for DOT 3 and has a higher boiling point than DOT 3. Many performance brake manufacturers recommend DOT 4 fluid.
DOT 5 is a silicone-based fluid, often referred to as a ‘synthetic fluid.’ It does not absorb water, and it won’t damage your paint, but it is slightly more compressible than DOT 3 and 4, so you might notice a bit of a spongy feel to your pedal. DOT 5 is usually a purple color so you really notice it’s different. If you use DOT 5, you don’t want to top off your master cylinder with anything but that fluid. If you do get some DOT 3 or DOT 4 in your DOT 5 system, you gotta clean out the whole system and start over. I use this in my autocross vehicle due to its high boiling point.
DOT 5.1 is a non-silicone poly glycol fluid, extreme duty for heat and longevity – think Formula One race cars. It technically can be mixed with DOT 3 and 4, but don’t.
So, which do you use? My recommendation on a street car with no track time is DOT 4, but you need to clean out the system or stick with DOT 3. A street car with occasional track time will benefit from DOT 5. If it’s a full race vehicle, probably still DOT 5, but go with your brake system manufacturer’s recommendation.
Bleeding your brakes is one of the few jobs in life that have a number of perfectly acceptable ways to do it. It’s important to note that all these methods are probably best done starting with the furthest bleeder valve and working closer to the master cylinder. In most conventional American cars, that means starting with the passenger-side rear, then the driver-side rear, then the passenger-side front, and last the driver-side front. Also, since fluid is moving from the master cylinder and out each bleeder valve, you need to keep an eye on the fluid level and add fluid as needed.
This is kinda like using the Force. I’ve never been bored enough to try this, but I’m told it does work well. Basically, open the reservoir, fill it, open the bleeder valve farthest from the master cylinder (using your clear plastic tubing and that container). When the fluid runs clean and without bubbles, close the valve and move to the next one. Super simple. I’ve never used this method, mainly because I’m not that patient. Maybe next time.
Open reservoir and fill it, put the cap back on to help avoid some mess if your helper pushes the pedal too quickly. This is a two-person job, so go find your sweetie and promise a long foot rub after the job is done. There will be one person in the car, the other at the first bleeder valve. Attach that clear tubing, have the person in the car pump the pedal in one smooth motion until firmish, then hold it down, holding pressure in the system. Crack open the bleeder valve until the helper tells you the pedal is on the floor. Close the bleeder valve and advise the helper to let go of the pedal completely. Do this process over and over on each wheel until fluid runs clean and without bubbles. When done with each wheel, make sure the bleeder is closed completely and recheck fluid level in reservoir.
There is an interesting alternative to the manual bleeding method that allows for a one-man job. You can find ‘solo brake bleed screws’ available at your auto parts store or through online suppliers. These replace your factory bleed screw with one that has a check valve built in. This allows you to crack the bleed screw open, then go to the driver’s seat and push the pedal yourself. When you allow the pedal to go up the check valve is engaged automatically so that no air gets sucked into the system. It’s an awesome idea, but not one that every enthusiast is sold on. If you give it a try, I suggest using the bleeder screws that have a brass plunger to shut off fluid flow. There is another type that has the check valve in the body, and these use a sealant that keeps air from getting into the system by seeping past the threads. The thread sealant normally allows for a couple of uses before it is no longer effective. If you only do this job once a year (or less) this is a good alternative.
Vacuum and/or Pressure Bleeding
For these types of bleeding you need a special tool, but you don’t need to beg for a pedal pusher to help. The tools allow for a one-man job. This is easy and probably the best for the DIY’er, and if you bleed your own brakes from time to time these tools can make the job fun, or at least more enjoyable than it was before. I bleed my brakes before every autocross or road race event, so I wouldn’t do the job any other way. I’ll offer some info here on two different types of tools.
When you look for one-person brake bleeding tool kits, you’ll probably find several that look like some kind of weird handgun. This tool is the type I typically use. There are really cheap ones, and really expensive ones. The more expensive ones will last longer than the first time you use them. This one can be used for both vacuum bleeding and for pressure bleeding.
The second type has a larger (half gallon or gallon usually) plastic reservoir and you use compressed air to force fluid into the master cylinder – or into the brake caliper if you want to go the other way. This type also works well, it’s a bit bulkier, not as portable, and has the potential to be messier.
I use the weird handgun type. I’m a big fan of autocross and use this tool to prep for each track event. I can take it with me and use it in the pits if necessary. I also use this method on my old pickup truck daily driver because I know it works well and can be done by one person without assistance. In this age of social distancing, that alone is a reason to buy the tool.
As a ‘hydraulics guy’ by background with a certification to prove it, even I will always take any anti-lock brakes to a pro for bleeding. The manifold and valves that make the ABS systems work has a complex network of porting and small orifices, so getting all the air out is a challenge compared to the brakes on our favorite muscle car or hot rod.