Lotus Elan

Brake bleeding sequence

PostPost by: martinbrowning » Sat Aug 13, 2016 8:37 am

Hi folks, Every day I take my St Bernard for a 3 mile meander ( have you ever met a Bernard with drive and focus?) across the fields. The route takes us past a premises run by an old boy/old school mechanic. Usually stop for a chat. Anyway, today he was standing with a bottle of brake fluid and rubber hose sticking out - proper brake bleeding kit.
The conversation turned to brake bleeding and one man kits. He swears by the traditional method, hence bottle and tube, but agrees that more advanced pressurised pump systems needed for modern cars with ABS etc.

But, he always bleeds in the sequence starting with the calliper CLOSEST to the master cylinder. I always thought that you started with the FURTHEST calliper.

Any thoughts?

Martin B
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PostPost by: Grizzly » Sat Aug 13, 2016 2:56 pm

I was always told to start from the most distant point and work back also go round at least twice.

But that said i guess every one has a preferred method, i've always had most joy with the 2 man three pumps and hold method of bleeding brakes myself. The one man type do work ok but require you to remove and grease the bleeders or it draws air back through the threads.

We have both Vacuum and pressure bleed systems at work and although the pressure system works really well i can't help feeling nervous every time i hook it up (only a rubber seal away from spraying fluid all over the engine bay etc) the idea is good with the vacuum but it's easy to run out of fluid and never seems as well bled as the old school method.
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PostPost by: dgym » Mon Aug 15, 2016 12:54 am

I start closest but I'm not sure it really makes a difference.
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PostPost by: The Veg » Tue Aug 16, 2016 2:13 am

dgym wrote:I start closest but I'm not sure it really makes a difference.


I'm inclined to agree. Each branch of the system must be fully purged at some point, so whichever you you do it, they'll all get done.
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PostPost by: pharriso » Tue Aug 16, 2016 3:26 pm

I've always done furthest to closest. A google search of the interweb yields some funny answers, the following 2 appear plausible:

1. Begin at the corner furthest from the driver and proceed in order toward the driver. (Right rear, left rear, right front, left front.) While the actual sequence is not critical to the bleed performance it is easy to remember the sequence as the farthest to the closest. This will also allow the system to be bled in such a way as to minimize the amount of potential cross-contamination between the new and old fluid.

2. going to the furthest gets the most air out right away......

& from http://mechanics.stackexchange.com/questions/27998/does-the-brake-bleeding-sequence-really-matter
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.
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