Brake pedal problem

[jetbox]

2.0 TDI quattro

No recent work on the brakes but I have no noticed that if you pres teh brake pedal with the engine running the pedal will slowly sink to the floor. With engine off it is fine.

I will try bleeding the brakes this evening but I suspect the master cylinder.

I do not understand why it only happens with the engine running though? Can any help.

thanks

Jetbox

[kenney]

This is not uncommon check another vehicle (TDI) and you will find it is the same

[jetbox]

Hi,

I bled the brakes tonight and it is just the same.

Interesting you say they all do that. We have had the car a couple of years and both my wife and I notice the pedal does not feel as sharp as it did...

Can someone else please try in their car (2.0tdi) and see if it does the same?

many thanks

[nevo]

Tdis will do that,as long as you have a firm pedal when you hit the brakes and it grabs the vehicle they're alright,when it hits the carpet then worry.The servo system can lose efficiency very slowly over time then suddenly you notice it.

[jetbox]

The pedal still feels strange and definately different to what it was> I drove about 40 miles today and the brake bleed has made no difference.

I appreciate the comments that all TDI's do that but can somebody with a 2.0tdi go and check with the engine running. Can you push the pedal right down to the floor?

I can not believe this is normal?

thank you

[phil miller]

we had a 2 lt TDI in today for a service and MOT and the brake pedal on that DIDNT creap down, i have known this in the past, but never on an Audi, are you sure you dont have a slight weep of fluid? have a look at all the calipers ive known them leak around the washers on the flexy hose banjo bolt and also around the piston in the caliper, if your unsure get your local garage to have a look at it on ramp, hope you get it sorted mate

[nevo]

If its going all the way down to the carpet get it checked over, thats to far.

[kenney]

If you have a leak then the pedal will also floor without the engine running,with normal braking you will not notice this (fault ???),only when you keep a constant hard pressure on the pedal,WITH THE ENGINE RUNNING,it will floor

[jetbox]

Hi,

There are no leaks or bulging flex hoses.

I don't undersstand why it does not do it with the engine off???

[patrickhorton]

When you press the pedal with engine off, is there still some pull left in the servo? You know what I mean when you get a few pumps at the pedal with the engine off and then the servo vacuum is used up so the pedal goes hard. Anyway, if the servo vacuum is used up, you would have to press the pedal as much as 3 times harder to get the same brake fluid pressures. Are you sure this is not why you can't repeat the problem engine off?

The only other explanation I could suggest is something wrong with the antilocking system. I presume this is totally disabled when the ignition key is off.

Definitely sounds dangerous to me. My A4 feels like any other car with normal amount of pedal travel engine on or off.

Good luck

Patrick

[jetbox]

Patrick,

Thanks for your reply.

The servo works fine, effective for a few pumps after ignition off etc.

I had the same theory as you regarding why I can not feel the pedal creep with engine off (eg more effort required) but I have tried this and I can push the pedal as hard as is physically possible and it will not creep down with power off.

Other option is maybe the abs as you say, I have no warning lights etc. I might try to see if I can pull the fuse to disable and then try it, hopefully to decide if its the abs module or the master cylinder....

[patrickhorton]

D'you get any success with the brakes, jetbox?

Can't believe I now have similar issue. Mine feels like there is a poor vacuum supply. With the engine idling, I can press the brake about three times and exhaust the servo as you would with the engine off. Once the pedal becomes harder (servo exhausted) if I hold my foot on fairly hard it slowly sinks towards the floor. However, it only goes as far as the point it would reach with full servo.
It's like the slow vacuum supply is slowly pulling the servo up to strength. I suspect a vac leak or the tandem pump is getting tired. If I rev the engine while pressing the pedal, the sinking speeds up (tandem pump turning faster).

Interested know your thoughts?

Patrick

PS for the record, twice the brakes have gone non-servo whilst driving - both times only a minute or so from start-up. Very scary.