Brakes explanation required. (3 Viewers)

[Clipgate]

This is a challenge I faced however not on a new or nearly fleet of Peugeot Fiat vans, but on two motorhomes of 7 year old vintage and one of 9.
I am a qualified motor engineer and lapsed MOT 3 to 7 tester.
I did not accept the fault then or would I now.
On both occasions I stripped the system, overhauled the master cylinder, replaced brake hoses and rear brake compensator, reset compensator as manual etc.
Following this minimal brake pedal creep certainly not to the floor.
I believe someone has hit the salient point, on a long downhill descent with components heating and possibility of braking degradation, I would not want to be in a heavily laden van or Motorhome with the pedal getting ever more closer to the floor.
Good technique is vital of course using engine braking reducing speed etc.
However when comes to shove a pedal that goes to the floor after prolonged application is not a situation I am going to be happy with.
As always it's a matter of degree, but never forget you only find out the results of an action or inaction after the event quite often

 

[Jackd]

Subject closed

 

[TheBig1]

Subject closed

Did you see my description in post 30?

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[Jackd]

Did you see my description in post 30?

I have seen that, yes thank you.
My question would be, why not use a less powerful servo unit. Thus not having the power to force fluid past the seals and back to the tank. Surly this would cure the “phenomenon” of diesel creep. Yey or ney?

 

[Jackd]

This is a challenge I faced however not on a new or nearly fleet of Peugeot Fiat vans, but on two motorhomes of 7 year old vintage and one of 9.
I am a qualified motor engineer and lapsed MOT 3 to 7 tester.
I did not accept the fault then or would I now.
On both occasions I stripped the system, overhauled the master cylinder, replaced brake hoses and rear brake compensator, reset compensator as manual etc.
Following this minimal brake pedal creep certainly not to the floor.
I believe someone has hit the salient point, on a long downhill descent with components heating and possibility of braking degradation, I would not want to be in a heavily laden van or Motorhome with the pedal getting ever more closer to the floor.
Good technique is vital of course using engine braking reducing speed etc.
However when comes to shove a pedal that goes to the floor after prolonged application is not a situation I am going to be happy with.
As always it's a matter of degree, but never forget you only find out the results of an action or inaction after the event quite often

Thank you clipgate
I too will not accept it just as the phenomenon of diesel creep. “Yeah they all do that mate”
Did you eventually manage to reduce the effect by re sealing the master cylinder?
Did you investigate the possibility of reducing the amount of vacume that the servo see’s?

I’m just intrigued now

 

[Clipgate]

Very simplistic description of the test faults.
Footbrake

A fail applies if:

• there’s insufficient reserve travel on the footbrake(ie, it touches the floor)
• the brake pedal rubber is excessively worn*
• incorrect operation of the servo assistance system
• unacceptable wear on the brake pedal pivot

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[Clipgate]

In my view the fault was cumulative, and given the age of components decided to replace or repair all.
I should have checked after each unit repair, however did not.
Gut feeling is not good enough in my view,
The vehicle designer know more than I, thus reducing the vacuum efficiency was not an option I considered, or would.
Even with the utmost care replacing / changing brake it's possible to leave small amounts of water in a component, lurking with Mal intent.
Care and attention to detail is my suggestion.
I remember a bus striking a wall at the bottom of a hill in Sussex, Compressor failure.
The compressor was 20 years old, the chassis manufacturers manual said inspect! The pump manufacturers Clayton said replace every five years, to the best of my memory.
No injuries fortunately, but could have been different.
Our vehicles are very poorly utilised, low mileages and long lay up. Thus chassis manufacturers Service schedule's are in my view inadequate for the use we place on them.
You may get the passion I feel about Motorhome maintenance

 

[Jackd]

In my view the fault was cumulative, and given the age of components decided to replace or repair all.
I should have checked after each unit repair, however did not.
Gut feeling is not good enough in my view,
The vehicle designer know more than I, thus reducing the vacuum efficiency was not an option I considered, or would.
Even with the utmost care replacing / changing brake it's possible to leave small amounts of water in a component, lurking with Mal intent.
Care and attention to detail is my suggestion.
I remember a bus striking a wall at the bottom of a hill in Sussex, Compressor failure.
The compressor was 20 years old, the chassis manufacturers manual said inspect! The pump manufacturers Clayton said replace every five years, to the best of my memory.
No injuries fortunately, but could have been different.
Our vehicles are very poorly utilised, low mileages and long lay up. Thus chassis manufacturers Service schedule's are in my view inadequate for the use we place on them.
You may get the passion I feel about Motorhome maintenance

I totally agree.
I won’t be experimenting with the vacuum rest assured.
I will however be replacing the master cylinder and testing again in the hope of reducing the effect rather than eliminating it

 

[dunnah01]

Somebody may have tried to bleed the system in the past but allowed air into the ABS Modulator. It may be worth bleeding the ABS unit to see if this improves the pedal feel.

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[gus-lopez]

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

Yes,here from MOT manual;

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.

How many tag axle owners are suffering badly with this?

It applies to any diesel vehicle not solely tags. My single axle Ducato coachbilt did it when stationary. My Transit does it when stationary. My Kia mpv did it.

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.

When you have stopped the handbrake should be applied every time. If you did that , & I had done when waiting for my gate to shut we would not have worried about something that doesn't exist when the brakes are needed in an emergency.
If you are riding the brakes it is wrong ,as above it should be cadence .Same as the abs works.

I would just like to get to the bottom of the problem to be honest

There isn't one.

However when comes to shove a pedal that goes to the floor after prolonged application is not a situation I am going to be happy with.

You shouldn't have a 'prolonged application ' if travelling down hill it should be cadence braking .

On both occasions I stripped the system, overhauled the master cylinder, replaced brake hoses and rear brake compensator, reset compensator as manual etc.

I replaced every part on my last coachbuilt as a matter of course due to age . Then I started to notice ,usually as I left home & waited for the gate to shut that the pedal was slowly sinking. I changed the master cylinder 3 times before it was pointed out to me it was diesel creep-& yes there wasn't anything wrong with the brakes when you needed them or did an emergency stop.


Easiest way to see whether you have a problem is during your regular test of the abs system does the vehicle stop?
Do the brakes, when applied to test the abs function, pulse the pedal under your foot & sometimes you can here the rat -a -tat-tat as the abs works to control ?
Find an empty piece of road wack it to 50,60,70mph then stand on the brake pedal with both feet.
If all the above happen there isn't a problem.

 

[Jackd]

Yes,here from MOT manual;

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.




It applies to any diesel vehicle not solely tags. My single axle Ducato coachbilt did it when stationary. My Transit does it when stationary. My Kia mpv did it.

When you have stopped the handbrake should be applied every time. If you did that , & I had done when waiting for my gate to shut we would not have worried about something that doesn't exist when the brakes are needed in an emergency.
If you are riding the brakes it is wrong ,as above it should be cadence .Same as the abs works.

There isn't one.

You shouldn't have a 'prolonged application ' if travelling down hill it should be cadence braking .

I replaced every part on my last coachbuilt as a matter of course due to age . Then I started to notice ,usually as I left home & waited for the gate to shut that the pedal was slowly sinking. I changed the master cylinder 3 times before it was pointed out to me it was diesel creep-& yes there wasn't anything wrong with the brakes when you needed them or did an emergency stop.


Easiest way to see whether you have a problem is during your regular test of the abs system does the vehicle stop?
Do the brakes, when applied to test the abs function, pulse the pedal under your foot & sometimes you can here the rat -a -tat-tat as the abs works to control ?
Find an empty piece of road wack it to 50,60,70mph then stand on the brake pedal with both feet.
If all the above happen there isn't a problem.

So at no point have you explained or questioned any of the technical reasons for this “phenomenon”. You have just accepted it.
I have never asked if it would pass an MOT nor have I complained that the brakes aren’t performing as they should.
I’m loving the way you test ABS. Both feet....... Awesome!
As for when to apply my handbrake and how to travel downhill, invaluable. Thanks

Like I said, best we close the subject. Thanks for all the input though.

 

[Vipar]

I have read every thread and still can’t see the answer.

Why is it called diesel creep when the brake system is nothing to do with diesel. Only brake fluid? Surely a call or email to Stuttgart would get an answer. I find the Germans happy to help one solve a problem.

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[TheBig1]

I have read every thread and still can’t see the answer.

Why is it called diesel creep when the brake system is nothing to do with diesel. Only brake fluid? Surely a call or email to Stuttgart would get an answer. I find the Germans happy to help one solve a problem.

because it is a problem only seen on big diesel vehicles. Petrol engines develop a vacuum and this is used to assist the brake master cylinder. Diesel engines specifically use a pump to achieve the same. It is this pump that causes the issue

 

[gus-lopez]

So at no point have you explained or questioned any of the technical reasons for this “phenomenon”. You have just accepted it.

I now know why .It is so that the absolute minimum of pressure is needed to apply the maximum force using the servo. I was only testing out some theories this morning & it is possible to brake easily using your big toe only , or an index finger.* The vast majority of people learning to drive & driving these days would think there is something wrong with a braking system like in older vehicles where you actually have to use greater force on the pedal to achieve the same braking affect

*& the abs is working ok.

 

[gus-lopez]

Why is it called diesel creep when the brake system is nothing to do with diesel.

As TheBig1 stated ,In a petrol engined vehicle when you lift off the throttle a vaccum is generated which is used as assistance for the brake servo.
A diesel vehicle does not create this vacuum. Therefore it is necessary for a mechanical, usually engine driven, vacuum pump to be running usually driven off the back end of the cam shaft/rocker shaft.
This generates full vacuum constantly.

To prevent damage to the seals, o rings & jointing a means had to be found to prevent the great pressure the servo can generate when people sit with their foot on the pedal or use it as a handbrake, & with the introduction of abs to prevent the unit being damaged by excessive continual pressure. To stop bigger, heavier vehicles with the minimum of pedal effort ,the manufacturers fitted bigger brake calipers and larger pistons. Also larger discs were fitted and also larger bore master cylinders. As a result the master cylinder displaces more fluid, but with less pressure. Leak by was /is the answer.
To enable this greater stopping power with minimal pedal force for much heavier vehicles double diaphragm servos became the norm.

Master cylinders with double operating shuttles with spring behind & between allowing leak by & pedal sink when pressure is maintained on the pedal.

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