Practical Automotive Maintenance: Brake Flushing And Bleeding

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Brake Flushing And Bleeding

08.55

00:00	Brake bleeding is one of the most fundamental skills required by anyone wanting to maintain their own brake system.
00:05	It's an extremely important skill to have and while it's critical to carry it out correctly for a properly functioning brake system, the good news is it's also very simple to do.
00:15	What we're trying to do when bleeding or completely flushing the brakes is remove the air and degraded fluid from the system and replace it with fresh fluid.
00:22	As discussed in our module on brake fluid, one of its key attributes is its incompressibility, which allows direct and accurate force transfer when the brake pedal is pressed by the driver.
00:32	When our brake fluid absorbs moisture, it becomes contaminated and begins to lose its incompressibility, often appearing dark and murky when inspected through the master cylinder reservoir.
00:43	This means that some of the braking force is lost and the same is true if air is introduced to the system.
00:48	This is when flushing with new fluid and bleeding out the air is necessary.
00:52	Before getting into it, it's important to remember that brake fluid is highly corrosive, so, we need to avoid getting it on our skin and on our paintwork, wear the right PPE and wipe off any spills with brake clean as soon as possible.
01:05	To do this, we'll need some new brake fluid that's compatible with the existing fluid and suitable for the application, a helper who can operate the brake pedal during the process, a spanner the correct size for the bleed nipples on our caliper and a bleed bottle with a suitably sized flexible clear hose.
01:21	Specific brake bleeding bottles are readily available, but any old soft drink bottle will work just fine.
01:27	The important thing about the hose is that it's clear, so we can monitor the state of the fluid as we go.
01:32	The end of the hose that attaches to the bleeder nipple also needs to be a tight fit in order to prevent leakage and mess.
01:39	If the fluid is in good condition and we're not changing the fluid type, then we'll only be looking to do a bleed instead of a full flush.
01:45	The steps in this process still apply.
01:47	We can just skip the parts that involve removing all the old fluid from the system completely.
01:53	The first step in the servicing process is to pop open the bonnet, locate the brake master cylinder reservoir and if we're doing a flush, empty it of the contaminated fluid.
02:02	This can most easily be achieved with a large syringe which can be found in most auto parts or tool stores.
02:08	We do need to be careful not to expose the inlet ports in the reservoir.
02:12	These are where the fluid is drawn into the rest of the system and if that happens we'll be sucking in air which is going to create a much bigger job when it comes time to bleed everything up.
02:20	We can now top up our reservoir with fresh fluid right up to the max line.
02:25	And if we're only doing a bleed, then we just need to top up the existing fluid to the same max line.
02:30	From here we need to access the brake caliper at each corner of the vehicle.
02:34	This is most easily achieved using a hoist, allowing easy access to the backside of the caliper which is typically where the bleed screws can be found.
02:42	If a hoist isn't accessible, we'll just need to jack the car up and drop it onto some axle stands on level ground, so that we can either lie underneath or remove wheels for easier access.
02:52	It's important to remember that we should never lie under a vehicle that isn't supported by the correct safety equipment.
02:57	We've already mentioned a brakes bleed nipple a few times in this course, so let's take a closer look.
03:02	This is a small component usually protected by a dust cover located near the top of each caliper that uses a small hollow screw with a tapered end to form a tight seal when closed.
03:12	Once opened using a spanner, brake fluid and air if present are able to pass through allowing us to both drain and bleed the system.
03:20	Some calipers have a single bleed nipple while others will have two or even four.
03:24	In a single piston caliper there will only be one nipple.
03:27	If there are two nipples and they're both positioned on the top of the caliper, this is to allow both sides of an opposed piston caliper to be bled.
03:34	If there are two nipples, but one is on the top and the other is on the bottom, we can ignore the lower nipple.
03:40	This is only there to allow the caliper to be mounted on either side of the vehicle where it would effectively be upside down and is redundant in its current position.
03:48	It's important to follow a specific sequence to flush as well as bleed the system, starting with the wheel farthest from the master cylinder and working closer.
03:57	On most right hand drive vehicles this means the left rear, right rear, left front and then right front.
04:02	For left hand drive cars this will instead be right rear, left rear, right front and then left front, but it's important to check the service manual for any vehicle just to be sure as some diagonal split systems use a different order.
04:16	For vehicles with an ABS system, a normal bleed or flush is usually fine.
04:20	With that said, if air has entered the ABS unit, for example after running the reservoir dry or replacing major components, many cars require a scan tool service bleed to cycle the ABS valves which will be detailed in the service manual.
04:33	At the first wheel in the sequence we'll wipe the nipple down with some brake clean, fit our clear hose over it and place the other end into the brake bleed bottle.
04:41	Our helper should press the brake pedal three times to build pressure and then hold it down firmly and call out something like down.
04:48	At that point we can open the bleed nipple about a quarter of a turn.
04:51	Fluid will flow through the hose and into the bottle and we can keep an eye on its condition as it passes through as well as if there are any air bubbles present.
04:59	Our helper must keep the pedal pressed until we close the bleeder.
05:03	It's important to keep the nipple open long enough to let a good volume of fluid out.
05:07	And if we're doing a complete flush, it's a good idea for the helper to actually give the pedal three to five pumps while the nipple is open which will expel much more of the old fluid.
05:17	Just keep in mind that we still need to always finish with a pump and then hold as we would with a normal bleed.
05:22	We can now close the nipple again and signal that we've done, so by saying up, telling our helper that they can now release the pedal.
05:29	Now, we can repeat the process, pump, hold, open, flow, close, release, until the fluid coming through is clean, clear and crucially free of air bubbles.
05:39	Once this has been achieved we know that each corner of the system has now been flushed and bled.
05:44	Throughout this process it's also very important that we keep a close eye on the master cylinder reservoir, topping it up regularly, so it never runs dry.
05:52	Allowing it to empty will introduce air into the system and we'll need to start all over again.
05:57	Now, all that's left to do is move on to the next wheel in the sequence and repeat the process until all four corners have been flushed with fresh fluid and bled of any air.
06:06	Once the flush and bleed is complete we can then ensure all bleed nibbles are securely tightened and cleaned off with a little brake clean.
06:12	After filling the reservoir back to max, refitting the master cylinder cap and wiping away any fluid spills, we can pump the brake pedal a few times and it should feel firm and consistent.
06:23	If it doesn't, then it's time to run through one more bleed on all four corners.
06:27	Before driving at speed it's always a good idea to test in a safe area, making sure the car stops straight and the ABS light is off.
06:34	We should also always dispose of the brake fluid properly as it's hazardous and should go to the recycling centre or an auto parts store that accepts it.
06:43	Lastly, labelling any open bottles we keep is always a good idea as brake fluid absorbs moisture once opened.
06:49	So, it's best to use a fresh sealed bottle for critical work and not store partials for too long.
06:56	With that covered, let's quickly run over what we've learnt in this module before moving on.
07:00	Flushing and bleeding a vehicle's brake system replaces old moisture contaminated fluid and any trapped air, both of which reduce braking performance.
07:08	Because brake fluid's incompressibility is essential for efficient force transfer, any contamination or air bubbles must be addressed promptly.
07:16	This process begins by extracting old fluid from the master cylinder, refilling it with fresh DOT spec fluid and then working through the brake circuit in the correct sequence, starting from the wheel farthest from the master cylinder until clean, bubble free fluid flows from each bleed nipple.
07:32	This can be done using the manual two person method where one person operates the brake pedal while the other opens and closes the bleed screws or with a pressure bleeder tool that draws fluid through without pedal use.
07:43	Throughout the procedure, it's crucial to keep the master cylinder topped up, close all nipples securely to the manufacturer's torque and ensure a firm brake pedal before returning the vehicle to service.