Hi All,

I think I know the answer to this question! However before I commit to spending cold hard cash, I just wanted to make sure.....

I have a MY2006 1.9TDi Elegance, for a while now I have noticed that if the brake pedal was pushed a little harder than normal when stationary, it would sink slowly towards the floor, if you then take your foot off the pedal and re-press it would be nice and hard once more. This is the same with or without the engine running. In all other respects, apart from requiring new fluid, the brake system appears to be in good condition and I have not had any moments when I have been left questioning the ability of the brakes to stop the car in complete safety. This has been in all conditions and in situations where hitting the centre pedal hard has been required.

As I need to change the brake fluid anyway I figured this was the best time to get to tackle this problem. From the research I have done the sympton above points to the seals on the master cylinder allowing brake fluid to leak past and that a new master cylinder is therefore required.

I have no objections to spending the money as this is the brakes we are talking about, however I prefer to spend money once where possible! If anyone has another suggestion to the cause of the problem or just wants to confirm my thinking is correct please chip in!

Many thanks

Do you ever have to top it up? If not it could just be too large a water content in the fluid and as water is easier to compress than brake fluid the pedal carrys on to travel until the system is fully compressed but you mite aswell check the system for leaks while your on changing the fluid.

Edited 28 August, 201213 yr by 07 vRS Taxi

You can't compress water.

The problem with water in brake fluid it that it can boil at the temperatures experienced near the pads/disks. You can compress a gas (steam) so it only becomes a noticeable problem when you're braking which might be too late at that point.

If you're stationary then it's not likely the fluid is hot enough to boil water unless you've just braked to come to a halt. Either way you don't want to mess around with brakes so you should get it checked quickly.

Could be a vacuum leak in the brake servo?

You say the brakes are normal apart when stationary with your foot hard on the pedal? The pedal can creep down in that situation (don’t know how excess yours might be). One thing to do to check your system is handbrake on, engine off, pump the brake pedal 10 to 15 times to remove any power assist from the servo & after that the pedal should be solid. If it still creeps down it could be air in the system or your master cylinder seal’s (providing you’re not losing any fluid).

Edited 28 August, 201213 yr by Chezlow

Hi All,

Many thanks for the responses:

Do you ever have to top it up?

No, the only time I have had to touch the fluid in the 2 years of ownership was to remove some following a pads change on the rear and then much later on the front.

Could be a vacuum leak in the brake servo?

I don't think so as I have good 'feel' in the pedal as opposed to it just being as hard as rock. Which I believe is the usual symptom of a the brake servo not holding vacum.

You say the brakes are normal apart when stationary with your foot hard on the pedal? The pedal can creep down in that situation (don’t know how excess yours might be). One thing to do to check your system is handbrake on, engine off, pump the brake pedal 10 to 15 times to remove any power assist from the servo & after that the pedal should be solid. If it still creeps down it could be air in the system or your master cylinder seal’s (providing you’re not losing any fluid).

Literally just tried this and writing this response from the car! After pumping the pedal as suggested there is no give, just a very hard pedal! I had read that a diesel engine may cause the pedal to sink due to a higher vacum pressure. It maybe this is the cause, or at least partially to blame.

It could be worth properly bleeding the brake system, including the bleed nipple that is on the side of the master cylinder itself.

I am now heavily leaning to a full bleed including master cyclinder bleed nipples as suggested and then take stock. It maybe this is all it needs, however I have never had a pedal give like this one does. However, I know the brake fluid is past its best and this perhaps should be my first port of call.

Thank you all for your help