Flushing the brake fluid system is a chore we should do every two to three years. But it isn’t as big or difficult a chore as it might sound; that said, it is possible to mess it up; like all jobs, there are steps we need to nail in order to get this right.
To flush a hydraulic brake fluid system, use a vacuum brake bleed tool to suck old fluid from each of the four brake bleed valves in turn while topping up the fluid reservoir with fresh fluid. At least two visits to each wheel will be required removing two fl oz on each visit.
In this post, you’ll learn the brake flushing tool options, including a homemade DIY option, you’ll learn what can go wrong and how to avoid it, and finally, you’ll learn the stepped flushing process.
Tools to Flush Brake System
Is flushing the brake system the same as bleeding the brakes? The answer is, for the most part, Yes. Flushing the brake system is really the same job as bleeding the brakes; the only real difference – we bleed for longer as we want to push all the old fluid from the system. The tools we use to flush and bleed brakes are one and the same; the main tool is a brake bleed kit.
There are a few brake bleed tool options; a semi-pro type that uses a compressor to pressurize the brake system via the brake fluid reservoir, a bottle and hose that can be homemade or bought, and a hand-operated vacuum pump that sucks the fluid via each brake valve.
For this flush guide, I’ll use the vacuum pump, and you can check out the model I recommend here on the Brake maintenance tools page.
You can, of course, use the humble bottle and hose kit; it’s a little more work, but it will do the job. You can easily make one yourself, and I did exactly that and wrote a post about it, and you can see how I did that right here.
If you choose to go the MacGyver route (bottle and hose), then check out this brake bleeding post where I use a bottle and hose; just remember, we are flushing, so instead of bleeding each brake three times in one sitting, we’ll be visiting each brake at least twice removing approx two fl oz on each visit.
The system is deemed flushed when we’ve pushed about 12 fl oz of fresh fluid through.
The tools and supplies you’ll need include:
- Lug nut wrench
- Axle stand
- Box/Ring wrenches (usually 8 or 9 mm)
- New brake fluid 24 fl oz (2 small containers)
- Brake bleed kit
- Paper towel
- Wire brush
- Some grease or vaseline
- Torque wrench
Brake Bleed Sequence
Most hydraulic brake systems prefer bleeding in a particular sequence. That sequence typically dictates bleeding paired brake lines. Brake lines are paired for safety reasons, each pair are independently fed from the master cylinder. Meaning if one circuit fails the other will still work.
Typically brakes are paired diagonally, rear left (RL) with the front right (FR) and rear right (RR) with the front left (FL).
You can easily identify how your brake lines are paired as the routing is marked on the ABS modulator.
I prefer to end my bleed sequence at the brake closest to the fluid reservoir and so I begin bleeding at the rear left (RL). Start at 1 then 2, 3, and end at 4.
Pre Brake Flushing Tips
Here we touch on what can go wrong when brake flushing so we can avoid them.
The process of flushing the brakes is not difficult, but things can and do go wrong, and if they do, you should know you won’t be able to drive your car. As soon as brake valves are open, they allow air into the system, and the brakes are compromised.
OK, so what can go wrong when flushing brakes?
Common problems include the following:
- Allowing the reservoir to run dry while flushing- Top-up reservoir after each wheel is complete
- Using the wrong type of brake fluid – brake fluid is categorized by DOT, and while DOT3, 4, and 5.1 shouldn’t be mixed, they may if you are in a pinch. However, DOT5 must not be mixed with any other type of brake fluid
- Using old brake fluid – always use new foil-sealed brake fluid; even brake fluid with a lid on attracts moisture
- Stripping old bleed valve – clean the valve with a wire brush and ensure you are using the correct size wrench
- Breaking off the bleed valve – if the valve looks old and crusty, know there’s a chance it could break
In this guide, I’ll use a vacuum bleeder. You can check it out here.
Prepare the flush kit as follows:
- Ensure the bottle is clean and dry
- Ensure the bottle cap and hoses make a good seal
- Ensure the vacuum pump creates a vacuum
The bottle will need to be emptied after each brake is bled, and a waste container will be needed for the waste fluid.
The brake flush process is as follows:
The objective of flushing the brake system is as you know to replace the old fluid with new. However, we must do so without allowing the system to run dry. In effect, we use new brake fluid to push the old fluid out.
This is a repetitive process; we’ll need to go around each of the four brakes at least twice until we pass approximately 12fl oz (one bottle of brake fluid) of fresh fluid through the entire system. At that point, we deem the system contains only fresh fluid.
The process is as follows:
- Pop the hood and fill the brake reservoir with the specified DOT fluid marked on the reservoir cap and refit the cap
- Remove the rear left wheel (RL) (use an axle stand or the wheel to support the chassis)
- Clean the bleed valve with a wire brush and spray threads with WD40
- Remove bleed valve rubber dust cap
- Add a smear of grease on the threads of the valve (helps seal threads)
- Fit the Box/Ring wrench
- Fit the bleeder kit hose
- Build vacuum to approx 25 inHg
- Open the bleeder valve by turning the wrench counterclockwise a quarter turn
- Allow the bleeder bottle to fill to approx two fl oz
- Tighten the valve and empty the bottle
- Clean the valve with brake cleaner
- Move to the front right (FR) brake and repeat after first refilling the brake fluid reservoir
- Move to the rear right (RR) brake and repeat after first refilling the brake fluid reservoir
- Move to the front left (FL) brake and repeat after first refilling the brake fluid reservoir
- Repeat the process once again, staring at the RL (1)
- Finally, top up the fluid to the full mark and tighten the cap
- Clean all bleed valves and replace dust caps
- Refit wheels and torque to spec in star sequence
With the bleed complete, the pedal should be firm. Now go around each wheel and, using a light, visually check each bleed valve is dry.
No need to remove the wheel; you’ll see enough through the wheel spokes. If you find a weep, go back and dry it and reinspect after pressing the brakes a few times.
If your pedal does not hold pressure and is sinking to the floor, you have a leak. Go back and check all brake valves.
That’s it brake flush is complete. Great Job!
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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 fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.