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Old 29th January 2019, 16:37   #51
grout20
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Originally Posted by marinabrian View Post
Takes about five to ten minutes per leg of the circuit John, so just under an hour, and yes the most you need to keep an eye on really is the master cylinder level

It replaces all bar about 40ml in the ABS modulator, so near enough really

Brian

Sounds good .... really appreciate the advice..

I think I'll start Plus-Gassing my nipples in anticipation of some warmer months to come.

Not sure if that sentence will get past the naughty word monitor but it's an honest statement!

Thanks again Brian

John
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Old 29th January 2019, 19:10   #52
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If you don't have a T4 handy, the safest way to bleed the brakes on the car is without the ignition turned on, and without pressing the pedal.
I'm surprised to see only one person "thanking" MarinaBrian for this very helpful and relevant statement. In case anyone has missed the point, what this means is that you don't need a T4 to bleed the brakes. Expecting cries of disbelief, I offer below some basic information derived from RAVE by way of explanation.

If you look at the ABS modulator's flow diagram for any wheel, brake fluid from the master cylinder flows through the inlet solenoid valve (normally open) and thence to the caliper. 'Normally open' means that the default state of this valve, i.e. with no electric current applied to it, is that brake fluid is free to pass through it unhindered.

The outlet solenoid valves (also in the caliper circuit) are 'normally closed' so no fluid or air can pass through them unless a wheel sensor activates the ABS (which would not be the case when renewing fluid or replacing failed external pipework).

So what we effectively have is simply a length of pipe from master cylinder directly to the caliper. That's it, and this applies to all wheels. It has to be like this because the ABS is a fail-safe system. So if the ABS has not been activated there is no possibility of air entering any part which requires T4 to remove it.

I haven't found any reference to turning on the ignition activating any valves internal to the ABS modulator for testing purposes.

Simon
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Old 29th January 2019, 19:21   #53
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I'm surprised to see only one person "thanking" MarinaBrian for this very helpful and relevant statement.......

Simon

I was tempted, Simon, but in the end found it too stressful so just could not bring myself to do it!

Addendum - following overnight reflection, the information is too useful not to acknowledge and on the whole he is a nice man. So done.

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Old 29th January 2019, 20:13   #54
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Originally Posted by SD1too View Post
I'm surprised to see only one person "thanking" MarinaBrian for this very helpful and relevant statement. In case anyone has missed the point, what this means is that you don't need a T4 to bleed the brakes. Expecting cries of disbelief, I offer below some basic information derived from RAVE by way of explanation.

If you look at the ABS modulator's flow diagram for any wheel, brake fluid from the master cylinder flows through the inlet solenoid valve (normally open) and thence to the caliper. 'Normally open' means that the default state of this valve, i.e. with no electric current applied to it, is that brake fluid is free to pass through it unhindered.

The outlet solenoid valves (also in the caliper circuit) are 'normally closed' so no fluid or air can pass through them unless a wheel sensor activates the ABS (which would not be the case when renewing fluid or replacing failed external pipework).

So what we effectively have is simply a length of pipe from master cylinder directly to the caliper. That's it, and this applies to all wheels. It has to be like this because the ABS is a fail-safe system. So if the ABS has not been activated there is no possibility of air entering any part which requires T4 to remove it.

I haven't found any reference to turning on the ignition activating any valves internal to the ABS modulator for testing purposes.

Simon
It could be that lack of thanks to Brian's post is because the forum consists of very many OLD folk who probably knew that already.

I can remember bleeding the brakes that way more than 50 years ago but have since got out of that habit.---
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Old 29th January 2019, 20:13   #55
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I haven't found any reference to turning on the ignition activating any valves internal to the ABS modulator for testing purposes.

Simon
The ABS ECU runs it's internal return pump briefly on engine startup, and additionally while the ignition is in position 2 without the engine started, it remains in diagnostic mode.

If a "fault condition" occurs, such as failure of one of the primary circuits, the associated shuttle valve will operate to minimise fluid loss, and the ABS ECU will determine an open bleed nipple and loss of pressure on any of the two primary circuits as a fault condition.

This can result in air being trapped in either accumulator which requires a secondary bleed using T4, the symptom of air trapped in this area is commonly described as sinking pedal syndrome.

So the safest course of action is not to have the ignition turned on while any of the hydraulic circuits are open, as to have no possibility of either of the shutoff valves operating, and simply isn't necessary under any circumstance.

Brian
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Old 29th January 2019, 20:17   #56
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An interesting story, but without a source link, unfortunately it's a "my mate Dave".

Brake fluid has 2 boiling points. We know it absorbs moisture over time, so it has a "dry" boiling point for new untainted fluid, and a wet boiling point for old fluid 2-3 years old. In the case of DOT4 brake fluid that will be in most of our cars, the wet boiling point is 60 degrees C lower than the dry boiling point. That's a huge difference.

When the brake fluid boils, any water in it turns to steam. Steam/vapour can be compressed, which means that effort at the pedal isn't reaching the brake caliper pistons and braking performance is reduced.

I'll continue doing mine every time I buy a used car, and every 2 years thereafter. Others may differ, as is their right.
Doesn't the water boil BEFORE the brake fluid ??? Thus moving the brake fluid from where it's most needed.
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Old 29th January 2019, 23:13   #57
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I've never investigated the other cars I own with ABS braking, the only others are the Freelander with a WABCO system, and the 416 tourer with.....well your guess is as good as mine

I shall take a look see when I get a chance, as I do have a Microcheck

Brian
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Old 30th January 2019, 00:40   #58
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Thanks for all the replies! quite a mixed bag of answers

I guess I will have a go at the gravity way of doing it as suggested. however, I will wait until the middle of summer. Way too cold at the moment.
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Old 30th January 2019, 08:58   #59
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The ABS ECU runs it's internal return pump briefly on engine startup, and additionally while the ignition is in position 2 without the engine started ...
It is important for readers to bear in mind that the operation of the return pump described by Brian above will have no effect whatsoever upon the fluid (and air, if present) in the hydraulic circuit from the master cylinder to calipers. This is because the pump is isolated from these circuits by the outlet solenoid valve which is normally closed.

It should therefore not be concluded that this brief operation of the return pump requires the use of T4 in any subsequent bleeding operation.

Simon
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Old 30th January 2019, 09:08   #60
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It is important for readers to bear in mind that the operation of the return pump described by Brian above will have no effect whatsoever upon the fluid (and air, if present) in the hydraulic circuit from the master cylinder to calipers. This is because the pump is isolated from these circuits by the outlet solenoid valve which is normally closed.

It should therefore not be concluded that this brief operation of the return pump requires the use of T4 in any subsequent bleeding operation.

Simon
But this part which you have omitted from your quote does have a bearing Simon the wording you have replaced with a "......" that the ABS ECU remains in diagnostic mode, which leads on to this other important omission

Quote:
Originally Posted by marinabrian
If a "fault condition" occurs, such as failure of one of the primary circuits, the associated shuttle valve will operate to minimise fluid loss, and the ABS ECU will determine an open bleed nipple and loss of pressure on any of the two primary circuits as a fault condition.

This can result in air being trapped in either accumulator which requires a secondary bleed using T4, the symptom of air trapped in this area is commonly described as sinking pedal syndrome.
So the answer is simple, leave the ignition turned off while you are bleeding the braking system, and you will have 40ml of old fluid left in the system, and no air introduced.

Brian

Last edited by marinabrian; 30th January 2019 at 09:13.. Reason: Spelling
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