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I tested another Passat TDi today and sure enough at TVM sys it does do that. My question now if why, what is different between the petrol and Diesel version other then the vac pump. Regards Peter
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Hi
A petrol can only supply the vacuum that is in the manifold a diesel is a mechanical pump and keeps pumping as the engine is running so as your foot is on the brake the pump will carry on pulling more and more vacuum to the servo so the pedal will keep going further and further down the longer you hold your foot on. There is no fluid passing any seals it is just more and more vacuum being applied to the servo.
Hope this helps you understand if not I will look up some VW service info when I get to work tomorrow for you.
Regards,
Merlin.
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Very good idea. However as there is a direct connection to the Master Piston from the pedal, assisted or not, then the fluid must be going somewhere or the flexi's are ballooning. Interesting
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Just as the vac pump on a diesel runs with the engine, the inlet manifold on a petrol engine is held below atmospheric pressure all the time the engine is running, with the throttle butterfly shut. The engine is just a big vac pump, therefore no difference.
The VAG tandem pumps on the PD engines do produce a very healthy vacuum - but so does a petrol engine being overdriven down a hill with the throttle closed. Anyone remember Ford vac wipers?
I'll have another play with the Superb and see if I can reproduce this "pedal sink".
659.
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Hi
The inlet manifold can only pull so much vacuum where a mechanical pump will keep pumping. Remember this pump has to be able to give a decent vacuum instantly to the servo so I capable of pumping a lot more them what a petrol manifold can give and this is what it?s doing.
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The hire fleet I work on includes about 30 VW LT's and the new Crafters.
Almost every 'one off' hirer complains about the brake pedal sinking to the floor. The fact is that the pedal goes down a fair old way if pressure is maintained, but 'reserve travel' is more than enough. It's easy enough to check if you really need to, just disconnect the brake pedal from the master cyl. (clevis pin) and measure the total pedal travel - reconnect and recheck !
I must say that regular hirers never ever complain about the brakes - they're used to them !
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Agree with merlin.
Surprised nc has doubts about it !
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>>Surprised nc has doubts about it !
I do as well,only explanation I can think of is seal materials are softer and turning back slightly,pistons cannot move without fluid moving somewhere under that pressure IMO....??
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>>this pump has to be able to give a decent vacuum instantly to the servo
Not really - servos typically are of the suspended vacuum type. By which, I mean that during application of the brake, atmospheric air is suddenly admitted to the pedal side of the servo diaphragm, rather than vacuum suddenly being applied to the master cylinder side - during brake application, the pump sees no extra flow and no extra load - it's only when you release the brake that the vacuum pump flows any air, to rebuild the vacuum on the pedal side.
There isn't a real difference between the diesel vac pump and the petrol engine (which effectively works as a vac pump when the throttle is closed).
If the pedal continues to sink until it hits the floor, then there's something wrong IMO.
If, however, the pedal just sinks a bit and then stops, definitely stops, then I could believe it's all OK.
Just because a large number of vehicles exhibit the behaviour doesn't necessarily make it right, or "as per design".
Number_Cruncher
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The pedal sinks about two to three inches.
But if they all do it, then logically it is by design.
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TourVanMan TM < Ex RF >
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are there any MOT testers here i am sure this is in the inspection manual as not a reason for it to fail. I used to be one one but not any more as my new dealership is not an MOT station. I will see it i can get anything off elsa (VW service info) tomorrow.
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There's some extra information here;
www.motuk.co.uk/manual_330.htm
There's even a special section on what to do if there is pedal creep detected.
Number_Cruncher
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new it was there Spot on in section C
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>>There isn't a real difference between the diesel vac pump and the petrol engine (which effectively works as a vac pump when the throttle is closed).
I've been thinking a bit more about this, and there is a significant difference at tickover, where the petrol engine may be able to make 21 in Hg of vacuum, where a total vacuum is about 29 or 20 inches. So, a diesel vac pump may be able to generate up to about 1.4 times the vacuum of a petrol at tickover. On its own, I don't think this is the full reason though - because typically, you can take brake systems past the point where the servo can add any further assistance, past the so-called "knee point" in the pedal force / brake fluid pressure relationship. Typically, the higher the brake flid pressure, the higher the radial sealing force between the seals and the bores of the cylinders - these shouldn't leak.
The best I can do for explanation is that when working with more extreme levels of vacuum, the servo takes longer to reach equilibrium, and develop full assistance - this last bit of extra assistance as the servo balances the load may feel like pedal creep.
I'm reluctant to believe that this situation represents some overpressure event for the brake fluid and there is some fluid leakage going on. If there's any leakage, the pedal will keep falling, and hit the floor, and be an MOT fail as per the MOT manual.
If there's no fluid leakage, and the pedal stops (albeit lower than you might have previously expected as described by none) with plenty of reserve travel, then the system must have reached equilibrium, and all is well.
Yorkiebar, merlin, I don't really think these cars or your advice is unsafe or wrong, and I hope that my questions and views aren't antagonising you. I'm just trying to understand what is really going on with these systems.
Number_Cruncher
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There's one other thing that may have a bearing on this question - are these servos suspended vacuum type, or suspended air?
I have never knowingly seen a suspended air type of servo, the suspended vacuum type is much more common. Do suspended air type servos even exist at all?
Number_Cruncher
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Just spotted this thread. This is a common characteristic of Diesels with a vacuum pump, not just VW's. Its not a nice characteristic.
I think the servo is suspended vacuum. When the pedal is applied there is some movement of diaphragm which results is slight loss of vacuum (increase in pressure) in the vacuum line. As the engine idles the vacuum builds back up and so pedal is felt to sink a bit further. It should only go so far and not touch the floor.
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>>When the pedal is applied there is some movement of diaphragm which results is slight loss of vacuum (increase in pressure) in the vacuum line.
That's credible. Then, the small displacement of the diesel vacuum pump takes a while to re-build the vacuum, wheras a petrol engine can build the vacuum much more quickly.
Number_Cruncher
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You actually have to work at it to get the effect. Constant pressure on the pedal, of just the right amount of force, will slowly force the pedal towards the floor, as much as three inches sometimes. Pump it and it comes back hard.
The symptoms are a dead ringer for air in the fluid.
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TourVanMan TM < Ex RF >
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We have 2 current diesels and it does not happen on them or any other diesel (incl A4 TDI) I have driven...
madf
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I bet it does if you know how to check for it.
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TourVanMan TM < Ex RF >
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I bet you're correct. I am also sure if I drink enough I'll see pink elelephants and one will play the bagpipes.
I just don't want to:-))
madf
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Well I've just checked my 2.5 year old 28k mile Skoda Superb (B5.5 Passat clone) diesel. It doesn't do it.
I am familiar with this effect and have seen it before on other cars, petrol and diesel. My money is still on internal master cylinder leakage across the primary piston seal into the recuperation chamber. I've noticed this effect mainly on older vehicles but it never seems to cause any trouble even under severe braking.
659.
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With all due respect 659 you are comparing it with a different fault/symptom.
There is a difference between engine running or not.
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>>you are comparing it with a different fault
Not according to TVM its exactly the same,i also think its not a servo problem but master cylinder!
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The engine was running - hot at idle.
659.
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If you suspect faulty master cylinder, even though brakes work as required, even under heavy severe braking, then I strongly suggest you have a new one fitted immediately.
I don't actually think this is the "fault" on the OP's post.
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