Possible brake master cylinder problem

[rayf3262]

Just tried the same test on my Skoda 2.0TDi, and by using pedal pressure in excess of normal braking, I can get the brake pedal to sink to the floor.....

 

[solarman216]

shows the holes in the plunger behind the disk to allow fluid through

 

[solarman216]

this is the rear plunger, much the same as the front, in normal operation the fluid trapped between the rear plunger and the front pushes the front plunger as well, in the event of say the rear brakes failing then the rear plunger travels till it contacts the front so maintaining front braking, should it be the front that fails then the plunger travels till it hits the cylinder bottom so keeping the rear operating

 

[Sweety]

Fantastic pic's Rick:thumbs replaced a few over the years on different cars but never opened one up to see what's inside or how they work:augie

 

[solarman216]

with the brake off the rubbers are just to the right of the small holes to the left of the larger ones, as the brake is pressed the rubbers pass these two holes and each circuit is sealed, further pressure applies the brakes

 

[solarman216]

scoring on plunger

 

[solarman216]

similar on disk

 

[solarman216]

good side of disk

 

[solarman216]

As far as I can see the disks are the possible reason for creep, the rubbers are not a snug fit onto the piston and the design of the rubber does not lend itself to increased grip on the piston, also the piston at this point is rough machined and cannot be designed to seal, so it is relying on the disks sealing the holes which in the condition these are in is not going to happen, I will be reassembling this cylinder without the disks as I think the rubbers will seal no problem, it will of course only go on my off roader, cannot do that sort of mod on a road going car, Rick

 

[solarman216]

Rick, I have massive respect for your technical knowledge and practical ability but this does seem to be a known characteristic of some diesel vehicles. Google "diesel creep" and you get thousands of results reporting the same symptoms. The MOT handbook seems to recognise it and says that it's not a fail unless the pedal reaches the floor as long as the pedal remains firm with the engine off.

I'm very happy to be proved wrong and I try hard not to talk/type crap.

Regards, Ian.

Sorry my "crap" reply was not directed at you, more the imbeciles that started this nonsense in the first place, just because there are thousands of faulty cars out there that the fault is possibly not bad enough to fail the MOT does not make it good or correct, where brakes are concerned this is purely nonsense, if your pedal sinks even half way it is faulty and should be rectified without fail, Rick

 

[solarman216]

Just tried the same test on my Skoda 2.0TDi, and by using pedal pressure in excess of normal braking, I can get the brake pedal to sink to the floor.....

You cannot exsert excess pressure on a brake pedal, if your pedal sinks you have a leak or a faulty system, I cannot however quote on ABS systems as these are very different, Rick

 

[Sweety]

Just a thought Rick would it be possible to reface the disk my rubbing it across some wet & dry paper on a flat surface or are the marks to deep for that

 

[solarman216]

Fantastic pic's Rick:thumbs replaced a few over the years on different cars but never opened one up to see what's inside or how they work:augie

Glad you like it, there is nothing too onerous in these cylinders, Rick

 

[kitchenman]

I plan to replace my master cylinder next week
If I still have creep where should I look
I am NOT losing fluid
How much DOT4 should I buy to replace the fluid which may be 21 years old?

 

[rayf3262]

You cannot exsert excess pressure on a brake pedal, if your pedal sinks you have a leak or a faulty system, I cannot however quote on ABS systems as these are very different, Rick

well it does have ABS. so maybe that is significant?

 

[solarman216]

Just a thought Rick would it be possible to reface the disk my rubbing it across some wet & dry paper on a flat surface or are the marks to deep for that

not really they are very thin and the surface finish on this sort of stuff should be mirror finish, you cannot see it in the pics but both disks have been deformed slightly into the holes in the piston, this looks like a basic design fault from Nissan, and probably a cop out on the real reason for the failure of the type that has been mentioned on here, pedals should not sink, ever, if they do something is wrong, OK in this instance it is unlikely to fail badly but fail it has, Rick

 

[solarman216]

well it does have ABS. so maybe that is significant?

very much so, it has a high pressure hydraulic pump that does the work for you, when you press your pedal you are in effect just opening a valve, but to give you "feel" there is some back pressure, Rick

 

[solarman216]

I plan to replace my master cylinder next week
If I still have creep where should I look
I am NOT losing fluid
How much DOT4 should I buy to replace the fluid which may be 21 years old?

new cylinder and you cannot have creep, would not mind betting that latest cylinders have been modified, you will need 1 litre of dot 4, and if you can bleed from all the bleed points to remove the old fluid, I say if you can because they may be seized, Rick

 

[kitchenman]

Thanks Rick
I am still somewhat puzzled that I get pedal creep with the engine on but not with it off, that is what suggested to me who has no knowledge on servos that perhaps the servo was to blame
Since this thread started I have tried several times to get the pedal to creep with engine off, from cold, warm and hot no creep,
With engine running and pressure on pedal, I have creep, turn off the engine creep stops even if I increase the pressure, I suspect the servo
After master cyl change I will try again

 

[Ogri]

Just to put the cat among the pigeons (again) I have found this page that suggests that newer diesel vacuum pumps are liable to "over assist" the pedal. In other words, while stationary, with engine running it is possible to achieve brake line pressure far above that which could be achieved in normal driving. If I'm reading it correct, the line pressure rose by 800 psi as the pedal crept downwards, suggesting no fluid leaks/bypasses but rather that the pump and servo combination could be forced to over assist the pedal and increase line pressure in proportion to the extra pedal travel.

Please don't shoot the messenger :hide:

http://www.nbsbrakesupplies.com.au/technical-information/d40-navara-creeping-brake-pedal.html

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