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I read that in general rear wheel drive vehicles have break line circuits that are rear / front split, and that front wheel drive vehicles tend to be diagonally split ((LF,RR),(RF,LR)).

How useful is this as a rule of thumb when changing break fluid? Is it pretty much always correct, or does one still need to verify the split in some way before changing the fluid?

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  • Did you ever find a satisfactory answer to the actual question you asked? I have the same question.
    – JakeRobb
    Commented Sep 10, 2023 at 16:44

3 Answers 3

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Edit: My answer was written from memory and was incorrect. Front/rear split systems remain in use to this day, mainly for rear-wheel-drive vehicles.

Best advice is to consult the manual on brake bleed order. Even when you know how the system is split, some manufacturers recommend non-obvious orders for bleeding.

US vehicle regulations don't even specify that a hydraulic brake system must be split, much less how it must be split.

I assumed the advantages of a diagonal split would mean nobody'd still use a front/rear split, but that's not the case.

There are some cars with redundancy on the front axle (one circuit is front + left rear, the other is front +right rear), e.g. Volvo has been doing this since the late 1960s. Other options are one circuit for front, another for all 4 wheels, or 2 circuits that serve 4 wheels each (Rolls-Royce do this IIRC).

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  • “These days” is awfully vague. Plenty of older cars on the road….
    – JakeRobb
    Commented Sep 10, 2023 at 16:45
  • By 1980, a diagonal split was the standard. Not many cars that old on the road.
    – Hobbes
    Commented Sep 10, 2023 at 17:14
  • Everything I can find indicates that both my 1987 Buick Grand National and my 2002 Camaro Z28 are front/rear. Despite that, the Camaro’s service manual specifies a bleed order that only makes sense if it’s diagonal. Is there a way to find out definitively, by inspection or test of the actual vehicle?
    – JakeRobb
    Commented Sep 11, 2023 at 18:36
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    Also, I don’t know where you live, but around here (Michigan), classic cars of the 60s and 70s are very common outside of the winter months. I rarely go a day without seeing one or two; often more.
    – JakeRobb
    Commented Sep 11, 2023 at 23:10
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When changing brake fluid you don't normally check the brake system's design; it doesn't really matter its layout. The procedure would be always the same: start purging the air from the farthermost wheel from the master cylinder, commonly the one at the rear, diagonally to the driver's seat. Then the other rear wheel. The again, the farthermost front wheel from the master cylinder, to finish with the one most close to it.

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    This isn't actually correct. Some manufacturers specify some pretty weird brake bleeding sequences. I was just reading up on Hyundai vehicles yesterday, which are RR, FR, LR, LF (IIRC). It definitely wasn't as you'v specified. Certain Audi's are different as well. I don't know why they are different, but they are.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 21, 2017 at 19:29
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Done 12,000 brake jobs every car in the world do it like someone said already. Done 1000s of cars never once looked it up and all of them worked out great.. When changing brake fluid you don't normally check the brake system's design; it doesn't really matter its layout. The procedure would be always the same: start purging the air from the farthermost wheel from the master cylinder, commonly the one at the rear, diagonally to the driver's seat. Then the other rear wheel. The again, the farthermost front wheel from the master cylinder, to finish with the one most close to it

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    This is incorrect information. While usually true to start from furthest, some manufacturers call for a different order. You can "think" you know everything, but when it comes down to it, the engineers who designed the vehicle easily know more then you. We should follow their instruction and not what we believe to be true.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 13, 2023 at 10:59
  • This "works out fine" because you get almost all of the air out of the lines. Sometimes, if there happen to not be bubbles at the critical points, you truly get all of it. And most drivers never stress their brakes to the point where one little bubble will make a big difference. My car sees track use with significantly elevated brake temperatures paired with very high line pressures, so I want ALL of the air out of my lines without leaving anything to chance.
    – JakeRobb
    Commented Sep 27, 2023 at 14:59

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