That pump operates at several thousand psi, forcing brake fluid through very small valves. Unlike the “solid column” system, ABS has a hydraulic pump. If you have ABS, air (and other contaminants) in the line becomes an even bigger problem. That single “soft spring” can cause the fluid level in the reservoir to drop even lower when the pedal is applied, which increases the chance of more air bubbles entering. If the level of fluid in the master cylinder gets too low, an air bubble can find its way into the line. That fluid comes from the master cylinder reservoir. That means the column needs to be marginally longer, which in turn means it needs more fluid. When the pads get thin the “solid column” has to reach further to apply the pressure being applied from the pedal. Boiling causes stream, and part of that steam will turn into air when it condenses.Ĭhanging brake parts like calipers will also allow air in the line. This is particularly true if you do a lot of hard braking or travel long, downhill grades on a regular basis. When moisture enters the line it reduces the boiling point of the fluid and can cause boiling. DOT brake fluid only has a life span of roughly three years. Well, there are a number of different ways. When this happens, your confidence in your car’s ability to brake might be a bit shaky. That’s what gives you the spongy feeling, and that’s what degrades the efficiency of your brake system. Air is “compressible.” When even a bubble or two enters the brake line, it’s like installing a soft spring in that solid column of brake fluid. When you press on the pedal you are pressing on a “solid” column of brake fluid which in turn causes the calipers to press the pads against the discs. In its simplest form, your brakes are a hydraulic system. How Does Air Enter the Brake Line in the First Place? First, let’s look at what exactly is causing your car issues. Some models require a very pricey proprietary ABS scan tool to cycle the pump and valves to purge the last of the air, but we’ll get to that in a minute. A newer car with ABS can get a bit trickier, and it can also be downright expensive. That’s the drill for an older car without ABS. You can fix the problem with a box wrench, a turkey baster, a plastic soda bottle, some tubing, and an assistant who is tall enough to reach the brake pedal. The good news is, learning how to bleed brakes is easy. What I've provided here is just an explanation as to why the rule of thumb exists regarding a standard order for bleeding brakes.Are your brakes dipping closer to the carpet than they should? Do they feel spongy? If you inspect the brake pads and they appear to be in decent shape, the “spongy” feeling is most likely caused by air in the brake line. However, when you are bleeding the 3rd closest, there is fluid flowing past the Second and 1st closest, meaning that you still need to bleed those when you're done with the third closest.Įdit: As pointed out in the comments, you should always refer to your manufacturer's instructions for the correct maintenance procedure for this and any other maintenance you perform on your vehicle. So, hopefully you can see via this illustration that if you are bleeding the 3rd closest wheel, there isn't any fluid flowing past the branch to the Fourth closest. Here is a rough illustration: # Rough Image There is no fluid that is passing that fourth branch, so there's less chance of air getting in there. This is because when you move from the furthest wheel to the second furthest wheel, you now only have fluid flowing past three of the four branches. When you start with the furthest wheel, though, you minimize the possibility that you miss air bubbles. You wouldn't even get that air out, because you've already bled that wheel and you think you're done with it. If you start with the wheel closest to the master cylinder and bleed it until there is no air, and then move on to the wheel furthest from the master cylinder (just as an example), as you're pumping the air out of that longest branch you could realistically get more air bubbles in the shortest branch. As you pump the brakes, the air will propagate down the hydraulic line, and randomly go down one of the branches towards one of the four wheels. At some point, the hydraulic line attached to the master cylinder will branch to each of the four wheels. If you have a lot of air right after your master cylinder for whatever reason, that air can travel to any point in the system.
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