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TDI 101 Got a simple/basic TDI question? Are you a newbie (new to the forums). Feel free to post your question here.

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Old January 13th, 2007, 18:49   #1
BugBoyOne
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Default Brake pedal goes to floor w/ engine running

I know this is a commonly asked question (yes I have read previous posts) but there seems to be no definitive answer. I am positive that I did not introduce air into the brake system.

Following a 2-yr pressure bleed brake fluid change, I noticed that, while the engine is running, I can keep a steady medium-high pressure on the brake pedal and it will slowly go almost to the floor.

With the engine off the brake pedal is firm and high - no sponginess whatsoever - one depression meets with immediate, firm and unyielding pedal feel - would seem to indicate no air in system, good master cylinder etc.

The brakes including the ABS work very well - just this strange spongy feel when you sustain pressure on the pedal while running. EG - firm pedal with engine off - start the car with foot on brake - after engine running, pedal will slowly depress almost to the floor. Some depression after startup indicates your booster is working, but this is a bit much.

All pads are good condition and hand brake is used and works well.

I re-bled the brakes via the manual method - no change. I am positive that I did not introduce any air via either bleed.

I can't be sure that the pedal has not always been like this - I just noticed this following a fluid change.

I read one post about using a VAG-COM to cycle/bleed the ABS pump: anyone have success eliminating this problem by this method??

This type of pedal feel seems common to TDI's with comments that it's normal and acceptable - can someone explain why this would happen and why some vehicles don't do this?

Thanks for any help.

2002 Jetta TDI

Last edited by BugBoyOne; January 13th, 2007 at 18:59.
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Old January 13th, 2007, 19:03   #2
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When the pedal slowly sinks to the floorboard, it indicates that fluid is leaking past the seals in the master cylinder. I think if you climbed down under the dash and looked where the master cylinder push rod enters through the firewall, you will see a trail of leaking brake fluid. Or, look at the brake booster and see if underneath the master cylinder brake fluid is leaking externally there.
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Old January 13th, 2007, 19:15   #3
Bootre
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could it be a vacum leak?
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Old January 13th, 2007, 19:26   #4
Dodoma
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What happened to pedal flooring? Got it fixed? Identified the problem so that if we encounter it, we can use formula of fixing.
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Old January 13th, 2007, 19:53   #5
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Vacuum has nothing to do with a brake pedal sinking. Vacuum only assists in the braking action in that the booster amplifies the amount of pressure the driver exerts on the pedal.
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Old January 14th, 2007, 00:07   #6
TNTKNC
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This problem happened to me after I had the stealership do a brake booster and ended up with a new master as well under warranty. Like you I was not sure as to whether it did this before I had the work done. I have re bled the system and still have the same feel. However the car still stops well and I have had no trouble with it. I would like to feel another TDI and see if the feel is the same. I still haven't found an answer. No one ever came up with a good answer for this.
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Old January 14th, 2007, 03:08   #7
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On EVERY SINGLE VW / audi vehicle I've tried and worked on, there has ALWAYS been a tendency for the brake pedal to SLOWLY go to the floor with the engine running and steady pressure on the pedal.

This is on cars from 1991 to 2005 (I have not driven or worked on cars newer than 2005, but I will think that the "problem" is still there).
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Old January 14th, 2007, 12:19   #8
BugBoyOne
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Thanks people. It sounds like this is an almost-normal VW brake pedal behaviour but why??

Here's my thinking based on good knowledge of manual hydraulic brake systems and scant knowledge of ABS:
  • There are no external leaks or brake fluid consumption
  • Engine-off testing would seem to prove that the master cylinder is good. I can stand on the pedal and it will not continue to go down - the master cylinder is not bypassing internally or leaking externally.
  • Booster problems would either make it more difficult to depress the pedal (e.g. vacuum leak - less assist) or maybe hold the pedal down, but not all the way down to the floor(?)
  • Pedal has a direct mechanical linkage to the master cylinder with booster as an assist only.
  • If the pedal ever goes to the floor, that means by definition that the pistons in the master cylinder have moved further into the master cylinder displacing brake fluid.
  • The master cylinder has only two output lines, likely to diagonally opposite wheel pairs (RF&LR, LF&RR) via the ABS module.
So, if there are no external leaks and the master cylinder isn't bypassing, having the pedal go to the floor, ever, means that brake fluid is getting pushed out into the system (ABS unit, lines, calipers). Where can this fluid be going??!!

Does the ABS pump behave like a hydraulic accumulator somehow?

I'm not desperate to "fix" this. I just want to understand it.
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Old January 14th, 2007, 13:11   #9
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Can the fluid leak into the brake booster chamber? If the fluid level is falling, its leaking, if not, its not leaking fluid (but I guess air can still leak in).

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Old January 14th, 2007, 15:47   #10
Zero10
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My golf doesn't do this, it's an 05 with 37,000km.
I have had this problem in 2 different cars in the past (a mazda and a porsche), and in both cases it was internal seal leakage on the master cylinder. Fluid was leaking from the pressure side back into the resevoir. This resulted in a sinking pedal with consistent pressure, but no external fluid leakage. Both times it was rectified with a new master cylinder, and both times repeated bleeding of the brake system didn't resolve the issue.
I just can't imagine it being anything else, but if people with new master cylinders are having this problem, then I guess I don't understand either.
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Old January 14th, 2007, 16:22   #11
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Default ABS the problem?

I have this "problem" with both the Golf and NB. No leaks, the resiviour never falls. I agree, the fluid from the pressure side must be going somewhere. The two choices are leaking back past the piston seal in the master cylindar and into the ABS pump somehow.

Once during a 2 year brake bleed I did allow the resviour to run empty in the Golf. It resulted in a spongy pedal that went half way to the floor before the brakes started to work. I did the ABS bleed and that solved the spongy pedal and brought the pedal travel back up. They felt great for a while

I never used to set the emergency brake because of all the problems that can cause in cold weather climates like New England. Now I do use it because it helps the rear adjusters work and improves pedal feel. Once per drive, when the car is stopped I also press really hard on the pedal and that seems to help with pedal travel and feel.

The Miata does not have ABS (it was an option) and it has always had fantastic brakes. Rock hard pedal, only 1" of pedal travel to engagement, and excellent modulation. I want our TDIs to have brakes like that. Every ABS car I have every driven has had mediocre pedal feel (soft).

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Old January 14th, 2007, 16:35   #12
BugBoyOne
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A passing master cylinder piston seal would explain this behaviour but why doesn't it pass with the booster out of the equation (i.e. with the engine off)? If the master cylinder is iffy, it should be iffy with the engine off as well.

I can generate very high brake system pressures with just my foot (no vacuum booster). Does the booster just generate higher still pressures and make any piston seal passing more evident?? As mentioned also, why do many behave like this after some kind of brake service, even after master cylinder replacements?

I will poll a few other TDI owners to see how common this pedal behaviour is.
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Old January 15th, 2007, 15:33   #13
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With the motor running the vacuum assist is working. My theory is without the assist you cannot generate enough pressure to cause the seals to leak and with the assist on you can.

I noticed a variation today on my way home. The pedal travel was very short. When I pressed hard the pedal did travel some but it did not continue to sink as I maintained pressure.

The sinking pedal could be an atribute of the ABS. When the motor is off so is the ABS. When the motor is on then the ABS computer is working. It could be that the ABS "sees" the pedal travel and does some brake proportioning.

All car braking systems have some form of proportioning valve. This valve is designed to maintain the proper front/rear braking bias. It also ensures that as you step on the brake pedal all the pads/shoes make contact before any significant brake force is generated. This was especially important on cars with drum brakes.

The ABS module may now be performing this function and may be allowing fluid to move to make up for some "error" in bias. When we take our foot off the brake the fluid is returned to the resevior.

This is all just a wild guess on my part.

Jeff
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Old January 15th, 2007, 19:44   #14
BugBoyOne
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I considered the ABS's role but I don't know enough about its details to explain this sinking pedal observation.

What I think is that the ABS reduces pressure to any caliper where the speed pickup detects no speed (locked wheel) during braking. It obviously has some other smarts to know to keep some pressure on while the vehicle, and hence all the wheels, are truly stopped and your foot is still on the pedal or else you would never be able to come to a complete standstill (how does it know that your actually sitting in your garage with your foot on the brake and not in a white-knuckle 4-wheel death skid on an icy freeway?? Hmmm - possibly another thread here)

The ABS can't feed fluid back to the reservoir directly, it can only push back against the master cylinder pistons with its pump. You can really feel the pedal kick upwards as the ABS starts working on slippery stops.

The ABS either allows the brakes to function as if it weren't there (as long as no wheel locks up) or it fights back to reduce individual caliper pressure as needed by pumping fluid back into the master cylinder (very small volume of an incompressible liquid would be required to reduce the caliper pressure). I don't believe that the ABS has some hidden fluid storage space that could account for the brake fluid volume displaced with the sinking pedal (unless it has some very strange pump). The ABS unit can't "bypass" and mysteriously make the fluid go somewheel else momentarily - it either goes to the caliper, maybe to the diagonally opposite wheel's caliper or back to the master cylinder. With the brakes on fully, all calipers are already clamped tightly and all line pressures are up. Where else could the fluid go??

The idea of the booster having enough oomph to cause the master cylinder piston seals to pass does explain what's observed but is hard to believe. I would think I could create just as much pressure with my foot and the engine off (the sinking pedal while running does only takes a steady medium foot pressure) yet sinking does not occur.

A mystery!
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Old January 15th, 2007, 23:31   #15
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It is probably the same as it was before you did the flush. I checked my pedal last night before I flushed the system and it would slowly go down near the floor if you are sitting in your garage with the engine running and kept very firm pressure on it. Feels the exact same way after the flush was completed. I think during normal operation you would be at a complete stop way before your pedal got anywhere near the floor.
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