Soft brake pedal & excess travel after fitting new front discs & pads

[MisterT]

I've just fitted new front discs and pads to my '08 2.0tdi 170 dsg estate.

Giving it a test run round the block I now notice the pedal is quite soft. I didn't notice a soft pedal before I changed the discs but the car stops reasonably well even though discs and pads are still bedding in.

When stationary with engine running the pedal is soft and travels a long way and then becomes firm. If I turn off the engine the pedal feels as firm as I would expect it to be and only travels a short way. If I apply the brake and then start the engine the pedal goes from being firm before start up and then sinks slowly before coming firm again after quite a long travel. I know this behaviour is what you would expect with brake servo etc but the pedal feel and excessive travel just doesn't inspire confidence or feel right.

I've done nothing to introduce air into the system, the only thing I can think that may have affected the hydraulics is pressing the pistons back into calipers to fit new pads. Checked hoses etc for leaks and nothing aparent.

I have visions of a duff master cylinder or something equally expensive!

Before I book it in with the dealer any helpful suggestions would be appreciated.

[Quatrelle]

Despite what you've said about just fitting the pads and discs, it all points to air in the system. What happened to the fluid when you pushed the pads back in?

You could try jamming the pedal down hard (before you switch off the engine) overnight, but bleeding the system sounds the best thing - I use an Eezibleed on my old MG, but I'm not sure it's suitable for a Passat.

Can't see that the m/cylinder would suddenly go duff - bit of a coincidence.

[Doctle Odd]

As above and another point. If you let the ABS pump go dry you'll need to cycle it to expel the air. I have used an easibleed on a B6 :)

[MisterT]

Quatrelle, You may be right I will try bleeding the system. There wasn't a huge amount of excess fluid, the reason I changed the discs and pads was due to heavy corrosion on the back face of the discs, the car has only done 45k miles and the pads were less than half worn but knackered due to the state of the discs.

[DSG4ME]

Did you simply push the rear pistons back or did you use a VCDS or Snap on caliper winder software.

[martin1810]

I've just fitted new front discs and pads to my '08 2.0tdi 170 dsg estate.

Giving it a test run round the block I now notice the pedal is quite soft. I didn't notice a soft pedal before I changed the discs but the car stops reasonably well even though discs and pads are still bedding in.

When stationary with engine running the pedal is soft and travels a long way and then becomes firm. If I turn off the engine the pedal feels as firm as I would expect it to be and only travels a short way. If I apply the brake and then start the engine the pedal goes from being firm before start up and then sinks slowly before coming firm again after quite a long travel. I know this behaviour is what you would expect with brake servo etc but the pedal feel and excessive travel just doesn't inspire confidence or feel right.

I've done nothing to introduce air into the system, the only thing I can think that may have affected the hydraulics is pressing the pistons back into calipers to fit new pads. Checked hoses etc for leaks and nothing aparent.

I have visions of a duff master cylinder or something equally expensive!

Before I book it in with the dealer any helpful suggestions would be appreciated.

It is a good idea to open bleed nipple before pushing/winding back calipers on abs cars. A complete pressure bleed may sort things out.

[RichardSEL]

Quatrelle, You may be right I will try bleeding the system. There wasn't a huge amount of excess fluid, the reason I changed the discs and pads was due to heavy corrosion on the back face of the discs, the car has only done 45k miles and the pads were less than half worn but knackered due to the state of the discs.

How did the discs get so corroded? Had the car been standing unused?

Change brake fluid, don't mess with bleeding old stuff, you'll be wasting the new that you're adding and suffer that sinking feeling again in a few months. Did mine at 52k (it's a time item of every two years irrespective of mileage) last year for first time from new '06MY as main agent didn't do it just "topped up". Difference was immediate: firmer pedal at nearly top of travel, slight sink is to be expected at engine start

[Doctle Odd]

Did you clamp the hoses? If you did they're probably damaged and will need replacing

[zollaf]

as martin points out, never push the pistons back without loosening the bleed nipple. if you did this to certain old vauxhalls you would destroy the master cylinder. some cars are fine about it, some are not. it seems to destroy the seal somehow, but not exactly sure why or how, but there we go, it happens. i always undo the bleed nipple regardless of what car it is, just to be safe.
you could try a bleed but i suspect you have destroyed the first stage seal and are now on the second stage seal which is for emergency use. :)

[MisterT]

Did you simply push the rear pistons back or did you use a VCDS or Snap on caliper winder software.

Its the front pads and discs I changed, not touched the rears as I dont have the necessary tool to plug into the OBD socket

[MisterT]

How did the discs get so corroded? Had the car been standing unused?

Change brake fluid, don't mess with bleeding old stuff, you'll be wasting the new that you're adding and suffer that sinking feeling again in a few months. Did mine at 52k (it's a time item of every two years irrespective of mileage) last year for first time from new '06MY as main agent didn't do it just "topped up". Difference was immediate: firmer pedal at nearly top of travel, slight sink is to be expected at engine start

The car has had a couple of periods of prolonged standing idle so guess that is the cause, if it were not for the corrosion I reckon there was another 40k miles left in the pads.

Will try pressure bleeding with new fluid as martin1810 suggests and see how it goes. Covered just over 100 miles in it today and the feel seems to be returning to 'normal' I guess the pads are just about bedded in now but would still like the pedal to be a bit firmer.


Thanks all for your replies, much appreciated.

[rexnanet]

Hi,

Did you solve the issue?

I got the same problem when changing the front pads on my A4 B5 except that they replaced the hoses too because they were a bit aged.

I now have replaced the master cylinder because I could floor the pedal but the issue returned to the original state: spongy until about half the travel then becomes hard.

This could be the first stage still damaged as (someone in a past reply...sorry :) ) mentioned.

Other option is that I might have air in the ABS block. They didn't bled it. I'm going to check if I have the option available on VAGCOM.

[RichardSEL]

Yes, ABS pump is the last component to be bled. Did you dump all the old fluid by flushing thru not with DOT4 but to spec. VW 501 14-B 000 750 and marked as such on the can.

There's no bleed procedure in VCDS. But you can have VCDS check ABS operation via a script. Procedure's in this vid in thread:
VWVortex.com - 01486 - System Function Test Activated in ABS (http://forums.vwvortex.com/showthread.php?5371931-01486-System-Function-Test-Activated-in-ABS)

[FrenchAudi]

I think one item that may be missed out when changing pads, is to check the seals on the pistons before pushing them back, and replace them if in any doubt.

If any dirt/corrosion/moisture has got inside the seals, the surface of the piston may be corroded, which will damage the seals when pushed back, and contamination might also get into the fluid.

[rexnanet]

Did you dump all the old fluid by flushing thru not with DOT4 but to spec. VW 501 14-B 000 750 and marked as such on the can.


The mechanic that changed the master cylinder said that he replaced it but I don't know for sure... I'll go and buy some to spec and replace it myself maybe. thanks for the reference!



There's no bleed procedure in VCDS. But you can have VCDS check ABS operation via a script. Procedure's in this vid in thread:
VWVortex.com - 01486 - System Function Test Activated in ABS (http://forums.vwvortex.com/showthread.php?5371931-01486-System-Function-Test-Activated-in-ABS)

I read in other forums that there is such procedure. But I don't know if mine has that option. Haven't got the chance to confirm it yet.

Also, I think I might have a small leak somewhere. The oil was topped at maximum level and now it's at about half.
Anyway, I think the cange was about 1 or 2 years ago so I'm going to replace the oil and check for leaks.


FrenchAudi[/COLOR];1027319]I think one item that may be missed out when changing pads, is to check the seals on the pistons before pushing them back, and replace them if in any doubt.

If any dirt/corrosion/moisture has got inside the seals, the surface of the piston may be corroded, which will damage the seals when pushed back, and contamination might also get into the fluid.

That's also a good clue, thanks. Not sure if they checked it...

[RichardSEL]

If you're working on an Audi A4 B5 then disregard my last. You're in the wrong forum, this is for VW Passat B6 2005-2010

[rexnanet]

If you're working on an Audi A4 B5 then disregard my last. You're in the wrong forum, this is for VW Passat B6 2005-2010
Yep, didn't notice that :) Ok, no problem. I'll check the right oil ref. or ask the guys on the official dealer.