15

We've all heard this tip - "start with the wheel furthest from the brake master cylinder and finish with the closest one".

Why? Does the sequence really matter? What exactly happens if we don't keep this sequence?

Does this tip even apply with modern systems, such as double circuit diagonally split brake systems? ABS systems?

  • 1
    I know on my 97 suburban with ABS, it still says to go furthest to closest. Having a hard time thinking of why since it has 3 lines from the ABS - FL, FR, and rear. – rpmerf Apr 8 '16 at 12:59
18

The sequence matters. 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. At some point, the hydraulic line attached to the master cylinder will branch to each of the four wheels. 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.

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. You wouldn't even get that air out, because you've already bled that wheel and you think you're done with it.

When you start with the furthest wheel, though, you minimize the possibility that you miss air bubbles. 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. There is no fluid that is passing that fourth branch, so there's less chance of air getting in there. Here is a rough illustration:

# Rough Image
                        -------
                       |       |
                       |       | <- Master Cylinder
                        -------
                           |
     Closest Wheel  -> ----|
                           |
                           |------- <- Second Closest Wheel
                           |
                           |
3rd Closest Wheel -> ----- |
                           |
                           |
                           |--------- <- 4th Closest Wheel

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. 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.

Edit: 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. What I've provided here is just an explanation as to why the rule of thumb exists regarding a standard order for bleeding brakes.

  • This works well for most systems. There are published procedures for every vehicle and most follow this general rule, some do not. I modify this by first bleeding any lines that are known to have air and then go around again following the published pattern. This method results in the least rework. – Fred Wilson Apr 9 '16 at 4:08
  • Great explanation! Though can this be applied to diagonally split and ABS systems? – I have no idea what I'm doing Apr 11 '16 at 6:55
  • 2
    The posted diagram does not represent modern braking systems. Modern systems have the LF and RR on one branch and the RF and LR on another. This seems like "old timey" wisdom that we've somehow held on to. – Mike Oct 20 '17 at 20:38
0

I did both calipers on my 98 Cavalier. I did the R.F. bleeding first as that is what my friends were saying. It worked fine, but I didn't believe it mattered. It seemed the driver side took a lot longer to get the air out, so I guess I did it right.

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