Brake System Design and Optimization: Brake Bleeding
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Brake Bleeding
13.06
| 00:00 | - In this section of the course, we're going to leave the theory behind and look at all the practical skills we might need to master when setting up, maintaining and optimising our motorsport braking system. |
| 00:11 | We'll start by looking at brake bleeding which is one of the most fundamental skills required by anyone wanting to maintain their own brake system. |
| 00:20 | 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. |
| 00:31 | In a motorsport context, brake bleeding is a maintenance item that needs to be carried out regularly with many high level teams bleeding the brakes after every session regardless of whether an issue has been reported or not. |
| 00:46 | What we're trying to do when bleeding the brakes is remove the degraded fluid from the system and replace it with fresh fluid. |
| 00:52 | It gets its name from the fact that it requires opening of the system at different points in order for the old fluid to be bled out. |
| 01:01 | As we remove the old fluid, we top up the fluid reservoir to ensure the system remains full and the new fluid replaces the old. |
| 01:10 | There are a number of different approaches to brake bleeding with special tools and processes available to help either speed up the job or reduce the number of people required to carry it out. |
| 01:21 | In this section we'll go through the tried and true approach that doesn't require you to invest in any special tools. |
| 01:28 | To do it, 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 your callipers and a bleed bottle with a suitably sized flexible clear hose. |
| 01:48 | Lastly we should always have some clean rags and brake clean on hand to clean up any small drips of brake fluid as we go and to clean up more thoroughly at the end. |
| 01:57 | Remember that most types of brake fluid are corrosive to paint so we quickly want to clean up any split fluid with a rag and brake clean as soon as they happen. |
| 02:07 | You can get specific brake bleeding bottles but any old soft drink bottle will work just as well. |
| 02:13 | The important thing about the hose is that it's clear so that you can monitor the state of the fluid as you go. |
| 02:19 | The end of the hose that attaches to the bleeder nipple also needs to be a tight fit in order to prevent any leakage and mess. |
| 02:27 | If the clear hose you have isn't the correct diameter, you can get a suitable short length of rubber hose that slips over the clear hose and locates well on the bleed nipple. |
| 02:37 | This can help solve the problem as the rubber hose can accommodate a wider range of diameters while still sealing. |
| 02:45 | The bleeding occurs at the brake callipers which will have bleed nipples fitted to them. |
| 02:50 | In order to successfully carry out the bleed the nipples need to be positioned at the highest point of the calliper. |
| 02:57 | Keep in mind that some callipers have a single bleed nipple while others will have 2 or even 4. |
| 03:03 | In a sinlge piston calliper, there will only be a single bleed nipple. |
| 03:07 | If we have two nipples and they're both positioned on the top of the calliper, this is to allow both sides of an opposed piston calliper to be bled. |
| 03:16 | If there are two nipples and one of them is on the top and the other is on the bottom, you can ignore the lower nipple. |
| 03:23 | This is to allow the brake to be mounted upside down relative to its current orientation and is redundant in its current position. |
| 03:32 | Any time we're carrying out the bleeding process, we want to begin as far away from the brake master cylinder as possible and work our way back towards them. |
| 03:41 | Depending on the way your system is plumbed. this may or may not be important but in many cases, this is the most efficient way to carry out the process of removing air bubbles from the system. |
| 03:54 | Regardless, it's a good idea to use the rule of furtherest to closest to help keep track of what we've done so far. |
| 04:02 | In this case where we have a right hand drive car, we want to begin at the left rear calliper, moving to the right then to the left front and then finally to the right front. |
| 04:12 | If you have callipers that have inner and outer bleed nipples on the top of the calliper, apply the same principle, begin with the outside before moving to the inside. |
| 04:22 | Step 1 is to jack up the car, place it on axle stands and remove the wheels. |
| 04:28 | This will give you better access to the bleed nipples on the callipers. |
| 04:31 | Remove the protective rubber caps from the bleed nipples if fitted and put them to the side. |
| 04:37 | If not, then give the nipples a quick wipe with a rag and some brake clean to remove any dirt or debris. |
| 04:44 | For step 2, check the brake fluid reservoir that supplies the master cylinders has sufficient fluid in it and top it up if required. |
| 04:52 | It should be at least at the full mark. |
| 04:55 | Step 3 is to move to the furtherest brake calliper and begin the bleed. |
| 04:59 | Remember to begin at the outermost nipple if there's more than one bleed position on the top of the calliper. |
| 05:07 | In step 4, we can fir the ring end spanner onto the bleed nipple, followed by the bleeder hose. |
| 05:14 | Make sure the other end of the hose is sitting in the bleeder bottle. |
| 05:17 | If our bottle has a hook, then we can usually hook it onto the suspension somewhere. |
| 05:22 | I like to do this to give myself a free hand throughout the process. |
| 05:27 | For step 5, we ask our assistant who's sitting in the driver's seat to press down on the brake pedal. |
| 05:32 | Throughout this process, it's critical to have clear communication between ourself as the bleeder and the assistant. |
| 05:40 | This is because to properly carry out the bleed, we need to work in a synchronised manner. |
| 05:45 | This process will be the same for every bleed cycle, the person that leads the communication is always the bleeder but clear communication in both directions is required. |
| 05:57 | The process always begins with the bleeder saying down. |
| 06:00 | This is the signal for the assistant to pump the brake pedal 3 times and on the final pump they need to keep a firm pressure on the brake pedal. |
| 06:09 | They should maintain this pressure until they're told otherwise. |
| 06:12 | At this point, the bleeder will open the bleed nipple using the ring end of the spanner. |
| 06:18 | It only needs to be opened a small amount and we don't want to open it any more than is necessary. |
| 06:24 | Usually a quarter turn will be sufficient and you'll know because when you have it open, the fluid begins to flow. |
| 06:31 | When the fluid flows out the nipple, the brake pedal will start to drop for the assistant. |
| 06:36 | They should keep consistent pressure on the pedal and allow it to drop. |
| 06:40 | As the fluid flows through the clear hose, we'll get a chance to check its condition. |
| 06:45 | If it's been overheated, it'll be a darker colour than the original clear to yellow shade when it's new. |
| 06:53 | If there's air in the system, you'll see air bubbles suspended in the fluid that will flow through the hose. |
| 06:59 | Another telltale sign of air being evacuated is that we'll often hear a slight clicking sound. |
| 07:05 | This is the air bubbles flowing out through the calliper and through the hose. |
| 07:09 | Once the brake pedal has dropped as far as it'll go, the assistant should say down. |
| 07:14 | The fluid will stop flowing through the bleed hose at this point and the bleeder should close the nipple. |
| 07:20 | The nipple doesn't need to be tightened hard, overtightening can easily damage the needle and seat at the bottom of the nipple or even snap it off. |
| 07:29 | It only needs a light tug to ensure the seat is properly closed. |
| 07:33 | Once the nipple is closed, the bleeder says up, this is to let the assistant know that they can release the pressure of the brake pedal, allowing it to naturally return up. |
| 07:43 | Once the pedal has returned all the way up, the assistant says up and this is to signal the bleeder that the bleed cycle is complete. |
| 07:51 | The order of this is critical. |
| 07:53 | The bleed nipple must be closed before the assistant releases the pressure of the brake pedal, this is why a 2 way communication is so important. |
| 08:02 | We repeat step 5 of the process until the fluid we're bleeding from the system is clean and contains no air bubbles. |
| 08:09 | Or the brake pedal firmness stops improving. |
| 08:12 | That might mean repeating this process 4-5 times before moving onto the next bleed position. |
| 08:18 | The other thing to be aware of throughout the process is that the brake fluid reservoir never runs dry at any point. |
| 08:25 | If this is allowed to happen, air will be drawn into the master cylinder inlet which will require an even more extensive bleeding process to be started again. |
| 08:36 | This means keeping an eye on the reservoir and topping it up as required. |
| 08:40 | It's normal to check this after about 4-5 bleed cycles but the capacity of every system is different. |
| 08:48 | This is something you'll quickly get a feel for as you go through the bleeding process. |
| 08:52 | Step 6 is to move onto the next bleed nipple and repeat step 5. |
| 08:57 | This may either be the inboard bleed nipple on the same calliper or the next calliper in the sequence. |
| 09:03 | Remember we're always moving from the furtherest point away from the master cylinder back to the closest. |
| 09:10 | Continue to move around the car until all the callipers and relevant bleed nipples have been bled. |
| 09:16 | At this point, as long as all the air is removed from the system and all the fluid coming out looks in good condition, and the brake pedal quality feels good, then we're done with the bleeding. |
| 09:27 | If you're bleeding the brakes to improve the pedal stiffness due to boiling the fluid, then the pedal feel should have improved significantly with a firmer feel. |
| 09:36 | Once we're happy the bleeding process is complete, we can move onto wrapping things up with step 7. |
| 09:43 | Go around and check all the brake bleeder nipples are sufficiently tight. |
| 09:47 | If you don't know what this should feel like, you can check the official manufacturer torque specs but after a while you'll naturally get a good feel for it. |
| 09:55 | Next we can move around each corner of the with our rag and can of brake clean, giving each nipple and the other areas of the calliper and suspension that have brake fluid on them are spray and wipe and then refit all the rubber dust caps. |
| 10:09 | Step 8 is to check the fluid level on the master cylinder reservoir. |
| 10:14 | This should be topped up to the max line then refit the reservoir lid. |
| 10:19 | Step 9 is to refit the wheels, drop the car back onto the ground and check the brake pedal again. |
| 10:26 | If everything feels good, then you're ready to go and test the car on the road or track. |
| 10:30 | Start out a little conservative on the first few brake applications to make sure everything feels good before moving onto harder stops. |
| 10:39 | One additional note here is that with some cars fitted with ABS systems, there can be another step required in the bleeding process. |
| 10:47 | This is due to a small amount of fluid that's kept separate within the ABS pump while it's not operating. |
| 10:54 | In the systems that have this, if a bleed is carried out without the pump operating then this fluid won't be flushed out during the bleeding process. |
| 11:03 | In cars with this style of pump, there's always a recommended procedure where the ABS pump is cycled during the bleed. |
| 11:11 | In some cases, this is only possible using a specialist tool to force the pump to run during the bleeding process, you should check the official recommendations for your make and model if you aren't sure if this applies to your model of car. |
| 11:26 | This can be either done with the dealer or an official servicing documentation. |
| 11:31 | The reason for the bleed is also an important factor when it comes to this ABS issue, if we're bleeding the system to remove air due to boiling of the fluid, in all likelihood, these bubbles will be within the callipers or at least very close to the callipers. |
| 11:47 | In this case it's unlikely that the fluid within the pump, needs to be bled at all and simply carrying out the standard bleeding process that we covered will be sufficient. |
| 11:58 | However in the case where you're completely flushing the system then it may be important to cycle the pump during the bleeding process. |
| 12:06 | In the end, if we aren't sure, it always pays to check with the manufacturer. |
| 12:10 | To summarise, bleeding the brakes is a critical process to make sure the pressure is effectively transferred from the master cylinders to the brake callipers and therefore ensure the brakes are operating correctly. |
| 12:24 | The key aim of bleeding the system is to remove old and degraded fluid which has absorbed moisture or been overheated and contains air bubbles. |
| 12:34 | While the bleeding process can vary somewhat depending on the layout of the braking system, the core steps are generally the same and involve working with an assistant who pressurises the system using the brake pedal while we bleed the damaged fluid from the nipples on the callipers. |
| 12:52 | Remember to begin as far from the master cylinder as possible and work towards it. |
| 12:57 | Always keeping fluid in the reservoir topped up and quickly cleaning any spillage of the corrosive brake fluid. |
