Garage Support

Post a Response

Skoda Octavia 1.9 TDi BKC 2005 - Sinking brake pedal #

Posted by Colin Hedger on June 27, 2017, 9:51 pm

Year of Manufacture *: 2005
( Optional ) Engine Code or CC: BKC

Hi all,
Have this Octavia 1.9 TDI with a slowly sinking brake pedal with the engine running.
Not spongey, but slowly sinking when held with a medium pressure, to within about 2 inches from the floor.
The brakes work fine, pull up well, ABS works, good servo assistance , no codes or warning lights and no fluid leaks.
Master cylinder has been replaced with genuine item and the problem, if anything, seems a little worse!
Brakes have been bled to within an inch of their life, manually, pressure bled and via the diagnostic machine.
All the pads and calipers have been dismantled, cleaned, checked and reassembled, the ABS unit has been removed and bypassed to eliminate internal leaks, and still the pedal sinks!
Any thoughts please?

Yours frustratingly

Colin.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Kevin Apps <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2589' width='330' height='300' border=0 style='border:none;'> on June 28, 2017, 12:14 am, in reply to "Skoda Octavia 1.9 TDI sinking brake pedal"
Kevin Apps

Colin
This known to happen with most diesel,s. if the pedal does not drop with-out the engine running, there is no fault.
Can,t even fail an MOT, you are on a wild goose chase, it is in the archives.

Kev.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by aidan birley <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2879' width='330' height='300' border=0 style='border:none;'> on June 28, 2017, 7:42 am, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Aidan Birley

Hi Colin

As Kev says, If it feel good on the road and does not creep down all the way to the floor it is normal.

Leave it running for a a few minutes with foot off, switch engine off then press the brake, if it dose not creep or touch floor it is fine.

A few years ago there was a special MOT notice about it and wording in the manual amended for 'high vacuum systems'

It becomes noticeable on diesels as the Vac pump is relatively slow in operation particularly to as it get to the high end of its range.

Aidan

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Damian Daize <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT5217' width='330' height='300' border=0 style='border:none;'> on June 28, 2017, 1:42 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Damian Daize

Hi Colin,
I tend to agree with the guys above on this one.
Does sound normal.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Ian Baxter <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT1560' width='330' height='300' border=0 style='border:none;'> on June 28, 2017, 8:06 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Ian Baxter

Hello Colin,

Just a general thought,

When manual brake bleeding I sometimes clamp off All the brake calipars , pump the peddle till firm, then remove only the clamp on the calipar you wish to start the bleed on, pump the peddle till happy with no air etc, lock off nipple and replace clamp and move on to the next calipar and carry out the same procedure until all calipars are done.

I sometimes find this works well on vehicles with two calipars per rotor, as it reduces movement in the clamped off calipars so the master cylinder only has to focus on one calipar at a time.

hth,
Ian.

P.S

Don't forget to remove all the clamps when you're done

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Colin Hedger <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT1335' width='330' height='300' border=0 style='border:none;'> on June 29, 2017, 7:57 am, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Colin Hedger

Thanks for all your replies, being someone who does'nt like to be beaten, you would not believe the amount of tme I have spent on this.
I wasted 3 car dealers time yesterday (payback!) and tried 3 other similar cars and yes, they all had a sinking brake pedal, (although maybe not as much as my car) so I'm starting to think this is just the way it is. I did not try a petrol car.
The more hoses you clamp, the slower the pedal seems to sink. With all 4 clamped it still seems to sink, all be it to an almost undetectable degree.
I would not be happy with this on my car, especially if I was holding it on the brake pedal on a hill.
If that really is 'just the way it is' can someone explain the theory behind this as there are are no compressable parts in a brake servo so pedal movement in equals master cylinder piston movement out!
Cheers

Colin.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Kevin Apps <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2589' width='330' height='300' border=0 style='border:none;'> on June 29, 2017, 1:42 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Kevin Apps

Colin
"I would not be happy with this on my car, especially if I was holding it on the brake pedal on a hill."

You would not need to press the pedal hard to hold it on a hill, try it !

As for "as there are are no compressable parts" there is a little give in all parts, the brake pressure is climbing due to the vac pump as per Aidan,s post even if you are not pushing harder on the pedal.

Kev.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Jon Morgan <iframe src='https://members4.boardhost.com/ukautotalk/info/user=ukat3190' width='330' height='300' border=0 style='border:none;'> on June 30, 2017, 1:17 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Jon Morgan

Info from Skoda bulletin.

The way in which the servobrake obtains a vacuum differs between petrol vehicles and diesel vehicles.

In a petrol engine, the vacuum is obtained through a direct connection between the brake servo and the intake manifold, with only one non-return valve along the line.

For diesel engine vehicles, a vacuum pump operated by the engine is required.

The difference between both systems lies in the capacity to intake air (air flow in litres per min) and the level of pressure provided by each system.

In the petrol engine, the flow is greater. This means that the time required to drain the volume in the brake servo is small. On the other hand, the vacuum pump used in diesel engines requires more time to drain the same volume of air, however, the vacuum pump is able to reach higher vacuum levels.

This means that following two or three consecutive brake applications for a diesel vehicle, if the brake pedal is held, an unwanted movement of the pedal occurs without increasing the foot force. This is due to the fact that after the brake has been applied several times, a large volume of the vacuum is exhausted and the vacuum pump needs a few seconds to replace it. This pedal movement is a sign that the system is resetting itself .

This situation can be felt when the car is at a standstill, and the driver has no feeling of deceleration, because in fact when the pedal is depressed as far as possible the driver is creating a braking pressure 4 or 5 times greater than that required.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Colin Hedger <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT1335' width='330' height='300' border=0 style='border:none;'> on June 30, 2017, 10:11 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Colin Hedger

Hi Jon,
I understand this description/theory (I think).
Thanks for that although I must say that in my 35yrs motor trade experience I have not come across these characteristics before - I'll be testing this theory in every diesel car from now on, in fact I tried this today in a Citroen Picasso Diesel and guess what - no sinking brake pedal!
But this pedal sinks regardless of whether there have been 2, 3 or no previous brake applications.

Colin

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Ian Baxter <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT1560' width='330' height='300' border=0 style='border:none;'> on July 1, 2017, 4:24 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"

Hello Colin,
Please bear in mind Not all diesel powered vehicles use an air vacuum servo controlled system for their power brakes

Ian

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Jon Morgan <iframe src='https://members4.boardhost.com/ukautotalk/info/user=ukat3190' width='330' height='300' border=0 style='border:none;'> on July 1, 2017, 5:43 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Jon Morgan

Hi Colin,
I can confirm that most VAG diesels suffer with the sinking brake pedal characteristic (having assisted many MOTs over the years). We are a VAG specialist and have seen this characteristic on VAG diesels many many times.

This is a definite characteristic of VAG diesel braking systems as confirmed by the copied and pasted text from the Skoda service bulletin.

Sorry to confirm you wasted time chasing down a characteristic of the system where no fault exists.

Regards,
Jon.

Re: Skoda Octavia 1.9 TDI sinking brake pedal - THE END

Posted by Colin Hedger <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT1335' width='330' height='300' border=0 style='border:none;'> on July 11, 2017, 10:18 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Colin Hedger

I've accepted this is a characteristic of the system now,but I still dont confess to fully understanding it!
Pedal movement in equals piston (of the master cylinder) movement out, so I'd love to know where that brake fluid is going!
Thanks for all your help anyway lads.
Exhaustedly

Colin Hedger

Re: Skoda Octavia 1.9 TDI sinking brake pedal - THE END

Posted by C Gordon <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT5449' width='330' height='300' border=0 style='border:none;'> on July 12, 2017, 7:19 am, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal - THE END"
Cambell Gordon

Fluid is being compressed to a very high pressure,If you did look the hoses might bulge a litle and the calipers will bend a bit more as the pistons try to escape.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by Kevin Apps <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2589' width='330' height='300' border=0 style='border:none;'> on July 12, 2017, 10:15 am, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"
Kevin Apps

Colin

"As for "as there are are no compressable parts" there is a little give in all parts,"

As i stated a little give in all parts NOTHING is 100% solid.
Put a steel ball under enough pressure and it will shrink in size.
Kev.

Re: Skoda Octavia 1.9 TDI sinking brake pedal

Posted by aidan birley <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2879' width='330' height='300' border=0 style='border:none;'> on July 12, 2017, 12:56 pm, in reply to "Re: Skoda Octavia 1.9 TDI sinking brake pedal"

Hi Colin

Good to see you are determined to get your head round this !

So hopefully to help

a) As said all parts will flex/ compress / give a little, if this was not the case as soon as the pads touched the disks there would be no more pedal movement be it gentle braking or emergency stop, clearly the peddle does move proportionally to brake effort so it is normal even within it working range.

b) Connect scanner to ABS and graph brake pressure, monitor pressure while applying brake. You will find that if you maintain constant fluid pressure no creep will be felt And when pedal creeps pressure will increase.

This has been noticeable on many systems for a good few years now, some more noticeable than others, I assume you are not an MOT tester or you would have come across it before.

I suspect they use these high vacuum systems so increase number of reserve assisted applications of the brake if engine stops or if driver applies & releases brake in quick succession. You may remember in the good old days when it was common to have a remote vac tank for extra storage, not needed with these systems.

Aidan

« Back to index

[ Garage Support ]