Brakes explanation required. (1 Viewer)

[Jackd]

Hi
I have a problem that I’m having trouble getting to the bottom of. It is going to require someone with in-depth knowledge to explain it to me.
The vehicle is 3ltr Ducato x250 tag axle.
My problem is, with the engine off my brake pedal is solid, even if I stand on it for 2 minutes.
As soon as I start up, ie the servo is working, any extended pressure on the pedal results in the pedal gradually sinking to the floor.
Keep the engine running and disconnect the servo vacuum, cures the problem again.

What’s actually happening?
Is the assistance from the servo pushing past the seals in the master cylinder?
It would appear to be a fairly common problem on larger diesel vehicles but NOBODY can explain what’s actually happening.
Because they use vacuum pumps, is the assistance too much for the cylinder?

Just so you know where I am in the process, all discs and pads are good. New dot 4 fluid throughout the system and bled properly. There is definitely no air in the system. There is no fluid loss anywhere on the system.
Tomorrow I’m going to take the brake lines off the master cylinder and blank off the outlets, start up and stand on the brakes again. This should at least help to determine where the problem lies.

Any help from someone with in-depth knowledge with this, would be very much appreciated .

 

[sonny]

Same on my sprinter not unknown on sprinters could try Mercedes forum I’ve not been able to get a definitive result myself still passes mot ok

 

[sonny]

Check out spongy brakes mhf

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[Jackd]

Unfortunately, you could end up in an argument with the wrong MOT tester.
I need to understand what is happening.

 

[mitzimad]

google deisel creep its to do with deisels having a pump to create vacumn unlike petrols which draw from the inlet manifold

 

[Jackd]

google deisel creep its to do with deisels having a pump to create vacumn unlike petrols which draw from the inlet manifold

Yes I understand that. I Have done the searches, but no explanations as to what’s happening.

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[Jackd]

I have just read this on another forum.

Read this from the gov MOT advice page............

Vehicles with petrol engines uses the inlet manifold to generate the vacuum for servo assistance.
Larger vehicles such as vans and 4x4s with diesel engines usually use a pump to generate the vacuum, Unlike the inlet manifold of a petrol engine, there is no vacuum relief with a pump. If excessive pedal pressure is applied when the vehicle is stationary (and the engine is running) as required for the MOT test the hydraulic pressure required to stop the vehicle will be grossly exceeded and fluid will be forced past seals that are between circuits.

The rare occurrence is known as diesel creep; and it is often incorrectly diagnosed as being caused by a faulty master cylinder.

The solution is to stop applying the excessive pressure.

If you can get the brake pedal to creep with the engine switched off and servo exhausted,
or actually under braking there is a serious problem that requires urgent attention!

A road test maybe an option to satisfy yourself, but if in doubt give the benefit of the doubt to the vehicle presenter.


This is all well and good and says that fluid is forced past seals. Where?
It appears that the larger the system, the greater the problem.
How many tag axle owners are suffering badly with this?

 

[mitzimad]

im not sure why your suffering badly if the brakes are working if they are not working its a fault somewhere

 

[Jimbohorlicks]

Has your van passed an MOT or failed on brakes?

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[Jackd]

im not sure why your suffering badly if the brakes are working if they are not working its a fault somewhere

Because it doesn’t inspire any confidence if you’re sat with your foot on the brakes and the pedal is gradually sinking to the floor.
How will this pan out coming off a mountain pass in the Alps next year? I would just like to get to the bottom of the problem to be honest

 

[Jackd]

Has your van passed an MOT or failed on brakes?

No no. Had the the van under a year and has never failed an mot. It is just very noticeable and worries me, so have been trying or cure it.

 

[Figaro]

I watched a video on youtube about this problem and after all usual checks, pads / hoses / bleeding etc, the guy found air being drawn in through one wheel cylinder. You may not have wheel cylinders but air could be getting in somewhere ?
I think you need a competant mechanic for this one.

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[Jackd]

I watched a video on youtube about this problem and after all usual checks, pads / hoses / bleeding etc, the guy found air being drawn in through one wheel cylinder. You may not have wheel cylinders but air could be getting in somewhere ?
I think you need a competant mechanic for this one.

Yes I saw that one. Like you said, no slave cylinders, just callipers.
We have found no air anywhere when purging the old fluid for the new.
You’re right, I need in depth knowledge

 

[2GB Racing]

I believe you will find that it is normal practice with diesels my 2015 Land Rover Discovery Sport has brake pedal creep and is supposed to have apparently ( they all do that sir ) says the workshop manager.
If static with engine ticking over and foot on brake pedal and just hold it firm it will slowly creep downwards, but driving it and use same pressure and ABS would kick in, Not an MOT issue as pass brake test with Loads to spare.

 

[Derbyshire wanderer]

The diesel creep can push the fluid back past the seals and this is considered normal on heavy vehicles due to the brakes being designed with enough power to work effectively no matter who is driving. The power you can apply when stationary is far in excess of what you are likely to try when on the move.
At MOT the brakes are tested to ensure that they both work on the rollers to the efficiency required as well as the appearance showing no signs of wear or corrosion etc.
The things that can cause excessive creep that can be controlled are the correct adjustment of drum brakes (much less common on later vehicles), the correct handbrake adjustment when fitted to disc brake callipers and more common on older vehicles is the swelling of the flexible brake hose lines.
Most likely though there is nothing wrong with your brakes.

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[Jackd]

The diesel creep can push the fluid back past the seals and this is considered normal

To dive into your answer a little more.
What seals is the fluid creeping past? Master cylinder?
Surly this must result in a lost of fluid or a gain in fluid in the reservoir?

 

[Geo]

You are looking for a cure or reasons for a problem that does not exist
Any escaping fluids returns back to the reservoir
and the system has no access to extra fluid to gain any at all
It is a diesel engine phenomenon that all mot testers are aware of.
The brake testing and pass or fail procedures are clearly available in the testers manual
You may argue that the test does not test the brakes as you are,
Thats because the brake testing method is laid out precisely and your method of testing produces incorrect results,

 

[Derbyshire wanderer]

To dive into your answer a little more.
What seals is the fluid creeping past? Master cylinder?
Surly this must result in a lost of fluid or a gain in fluid in the reservoir?

Its the master cylinder seals. Look into how they work and you will see that when the pedal is up the seals are behind a port that links the brake lines to the reservoir.
As the pedal is pressed the port closes to now make the piston and seals produce the line pressure. Any leakage past the seal will now get back to the reservoir which is not under pressure.
There is no overall loss or gain of fluid.

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[Jackd]

You are looking for a cure or reasons for a problem that does not exist
Any escaping fluids returns back to the reservoir
and the system has no access to extra fluid to gain any at all
It is a diesel engine phenomenon that all mot testers are aware of.
The brake testing and pass or fail procedures are clearly available in the testers manual
You may argue that the test does not test the brakes as you are,
Thats because the brake testing method is laid out precisely and your method of testing produces incorrect results,

Many thanks for your input
When you say I’m trying to solve a problem that doesn’t exist, forget the MOT
whilst the the pedal is travelling downwards I understand that the maximum brake pressure is being applied to the system plus extra which is what is causing the pedal to sink. However when the pedal reaches the floor, what is the state of breaking at that point? Release the pedal and re apply?
From my understanding, the reservoir is effectively a header tank and isn’t designed to be an expansion tank for the master cylinder.
In effect, what you’re saying, is that the servo is too powerful for the master cylinder thus pushing past the seals in the master cylinder back to tank. Have I understood this correctly?

 

[Portland]

I have not looked at all the posts but what you have is a brake system designed for 4 braked wheels, In all vehicles you have whats known as hydraulic leverage, that is the amount of fluid from the master cylinder to 4 wheels, so 1 to 4 you have 6 wheels using the same master cylinder feeding 6 brakes, so 1 to 6 you have increased the hydraulic leverage and to attain this the master cylinder moves more fluid so, longer pedal travel. Make sure the brakes are good, worn pads often stay away from the discs and need more movement. the reason the brake is solid when the engine is not running is because it has no assistance from the sevo, unless its mechanically operated it use's a second master cylinder to increase brake effort so long pedal travel. If the pedal has reserve travel when you brake it should be ok.

 

[ambulancekidd]

Every one of our Mercedes & VW ambulances did this, I was told by many mechanics that its a characteristic & therefor not a fault.
None of them leaked fluid & they all passed MOT's with no advisories.
None of our Transit ambulances did this though.

My own 2007 Sprinter did this right from the day I bought it.
Diesel creep was the answer I found when researching it, there was never any leakage from any of the vehicles mentioned here & MOT testers are a savvy bunch & wouldn't fail a vehicle for that reason.

Seems like you'll have to learn to live with it.

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[ambulancekidd]

the reason the brake is solid when the engine is not running is because unless its mechanically operated it use's a second master cylinder to increase brake effort so long pedal travel. If the pedal has reserve travel when you brake it should be ok.

There is a vacuum operated pump on all diesel vehicles with servo assisted brakes, its not a second master cylinder.
When stationary the brake pedal can be pressed right to the floor which naturally worries owners but the braking system is in perfect working order.
It doesn't happen when travelling & using brakes, even during emergency braking.

 

[chaser]

I have always had diesel vehicles and every one of them did this, never even thought about it and never had a vehicle fail it's mot for it. And also never had any brakes fail because of it.

 

[Jackd]

There is a vacuum operated pump on all diesel vehicles with servo assisted brakes, its not a second master cylinder.

Correct

When stationary the brake pedal can be pressed right to the floor which naturally worries owners but the braking system is in perfect working order.

Sorry, but Wrong

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[TerryL]

Because it doesn’t inspire any confidence if you’re sat with your foot on the brakes and the pedal is gradually sinking to the floor.
How will this pan out coming off a mountain pass in the Alps next year? I would just like to get to the bottom of the problem to be honest

Very easy. You MUST NOT IN ANY CIRCUMSTANCES "ride" the brakes in a long hill descent - it will rapidly lead to brake fade and total failure. Keep up the pressure and the fluid will boil - THEN the pedal will go straight to the floor. Sorry to be so emphatic but a very good friend died in the crash trying to do this to stop a runaway vehicle - however that's another story.

The correct procedure when making a long descent is to keep your speed low and use a low gear; someone said use the same gear you would use to climb the hill but even that might be too high. Forget about traffic behind you - they don't have your problems. Use your brakes to keep the engine from over-revving but press then release the pedal to give the brakes a chance for cooling between applications. If they still overheat then STOP and let them cool down properly - then when you get home have the braking system checked. I managed to warp the front discs by over zealous use of the brakes in mountainous areas. This is why you'll often see trucks crawling down steep hills - although many have retarder systems that don't directly involve the brakes, if they are at max loading (or maybe higher?) they'll need the brakes as well.

If your vehicle passed the MoT at a reputable testing station, and also judging from the replies above, there is nothing wrong with your brakes.

 

[Jackd]

Diesel creep was the answer

Correct
As already explained, Im Looking for the technical answer.
The only explanation so far is that fluid is being forced past the master cylinder seals back to the reservoir, thus allowing further travel in the pedal.
This can only suggest that that the servo assistance is too powerful for the master cylinder.
A lesser amount of vacuum perhaps?
I wonder why the super brains of the automotive world haven’t come up with the answer.

 

[Jackd]

Very easy. You MUST NOT IN ANY CIRCUMSTANCES "ride" the brakes in a long hill descent - it will rapidly lead to brake fade and total failure. Keep up the pressure and the fluid will boil - THEN the pedal will go straight to the floor. Sorry to be so emphatic but a very good friend died in the crash trying to do this to stop a runaway vehicle - however that's another story.

The correct procedure when making a long descent is to keep your speed low and use a low gear; someone said use the same gear you would use to climb the hill but even that might be too high. Forget about traffic behind you - they don't have your problems. Use your brakes to keep the engine from over-revving but press then release the pedal to give the brakes a chance for cooling between applications. If they still overheat then STOP and let them cool down properly - then when you get home have the braking system checked. I managed to warp the front discs by over zealous use of the brakes in mountainous areas. This is why you'll often see trucks crawling down steep hills - although many have retarder systems that don't directly involve the brakes, if they are at max loading (or maybe higher?) they'll need the brakes as well.

If your vehicle passed the MoT at a reputable testing station, and also judging from the replies above, there is nothing wrong with your brakes. After that there can only be one fault......................

WOW
in asking for some help on “diesel creep” technical knowledge from somebody with more knowledge than myself, I have managed to induce a driving lesson ending in an insult.

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[TerryL]

WOW
in asking for some help on “diesel creep” technical knowledge from somebody with more knowledge than myself, I have managed to induce a driving lesson ending in an insult.

Sorry -my apologies. In no way was it intended as an insult. I've edited that comment out.

If anybody else thinks I've gone over the top, trying to give advice/help to a relative newcomer, I'll delete the whole post.

 

[Geo]

Correct
As already explained, Im Looking for the technical answer.
The only explanation so far is that fluid is being forced past the master cylinder seals back to the reservoir, thus allowing further travel in the pedal.
This can only suggest that that the servo assistance is too powerful for the master cylinder.
A lesser amount of vacuum perhaps?
I wonder why the super brains of the automotive world haven’t come up with the answer.

To what?? see line one in bold

 

[TheBig1]

The image above shoes how a master cylinder works

Press the pedal and the pushrod moves the piston forward, closing the port to the reservoir. The fluid in this now closed system compresses and operates the brakes. With diesel fade (creep) the pressure produced overloads the O ring seals on the piston. Fluid bypasses the cylinder and returns to the reservoir. Release the pedal and the piston returns, allowing fluid from the reservoir back in front of the piston

I have not described the dual system to keep the explanation simple, but it works the same way

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