Latest issue.. awful brakes

[JonBoi25]

Had vagcom on the s3 yesterday and nothing really came up to pin point the issue.

I'm about to take another scan

I've checked the vac line and replaced the one way valve. No change

The main concern I have is if I'm driving along at any speed and put the brake on hard if I keep it down and press the accelerator the throttle overdose the brake and I accelerate... seems very dangerous to me ?

Surely brake signal overrides all else?

 

[S3AMJ]

I’m just trying to think when you would be braking hard to have the need to put your foot anywhere near the accelerator pedal.....

If you want to accelerate, simply take your foot off the brake.

I personally have never done it so don’t know if my car would react the same way.


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[ADs3]

Trying to heel & toe?.......I've heard something like this before & dont think it's a fault with the brake system......it's something to do with how the system is designed. Have a search I'm sure it's been discussed before

 

[JonBoi25]

Trying to heel & toe?.......I've heard something like this before & dont think it's a fault with the brake system......it's something to do with how the system is designed. Have a search I'm sure it's been discussed before

Yeah cheers mate.
I spent a while last night looking over similar posts. But the issues they've had I've checked out and all seems okay but perhaos I'm missing something.

In my mind the brake should always cut the throttle unless the remap has this feature removed for heel tow reasons. That in mind the brakes are non existent. My reason for looking into this.

If I plant the throttle and try to brake straight after I have nothing for a good few seconds. To me this points at the system losing vac. Leak etc

 

[JonBoi25]

I’m just trying to think when you would be braking hard to have the need to put your foot anywhere near the accelerator pedal.....

If you want to accelerate, simply take your foot off the brake.

I personally have never done it so don’t know if my car would react the same way.


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I've got an issue with the brakes in general. No fault codes nothing. The brakes are awful to the point if I start to boot it to over take then need to slow down and pull back in they are very dangerous and I could end up going into the back of someone. Of course I dont drive like a tool it's just always good to have things working safely as they should.

I just observed today while driving that if for whatever reason I was trying to stop (hold down the brake) I could still press the throttle and the car would accelerate hard like the brakes werent even on. My previous s3 I'm pretty sure would kill the throttle if you go near the brake. (Brake should override all). If for some reason the throttle stuck on I'd be screwed...

 

[S3AMJ]

I've got an issue with the brakes in general. No fault codes nothing. The brakes are awful to the point if I start to boot it to over take then need to slow down and pull back in they are very dangerous and I could end up going into the back of someone. Of course I dont drive like a tool it's just always good to have things working safely as they should.

I just observed today while driving that if for whatever reason I was trying to stop (hold down the brake) I could still press the throttle and the car would accelerate hard like the brakes werent even on. My previous s3 I'm pretty sure would kill the throttle if you go near the brake. (Brake should override all). If for some reason the throttle stuck on I'd be screwed...

Hmmm in that instance, I would check the VAC line like you’ve pointed out. Also check the none return valve (check valve) I’m not entirely sure about the servo on these cars, but on Jap cars I’ve owned in the past they have a diaphragm which can split stopping it from holding pressure to brake. Might be worth something to investigate....


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[mid lid]

If the car is on boost you will run out of vac servo assistance

 

[Stickystuff]

If the car is on boost you will run out of vac servo assistance

If the one way valve is working correctly then no this will not happen. I also think there is a vacuum pump to compensate? That might just be on Diesels.



Also with the brakes you can’t diagnose using a computer as the fault you’re experiencing sounds mechanical.

If your pedal goes to the floor then it’s likely a hydraulic leak either externally or internally.
Does it go to the floor or feel like it’s loosing pressure if you have the engine off? And pump the pedal so that any vacuum left in the booster is removed, the pedal should feel pretty hard. If it still feels soft or goes down then I’d check next:

I would check your brake fluid level first, if it’s going low then it’s going to be leaking fluid. Check under the master Ok the brake booster for the paint bubbling. If the paint is bubbling/peeling then the master cylinder is at fault, or atleast to start with.

Next check around each caliper and see if there is any brake fluid on them or dripping on a wheel etc. That will show up a faulty caliper.

You could try having a friend sit in the car and pressing on the pedal and you look around the hydraulic components whilst they are pushing on the pedal. It could be a hydraulic hose split if it’s the original rubber hose then they are 18 years old and highly likely to be perished. Or on the other hand it could be one of the metal pipes, the originals from the factory are steel and can corrode again causing a leak.


If there are no leaks then it could be a stuck caliper/pads etc.

I have experienced before(not on an Audi) where the brake pedal would go down really quickly unexpectedly it turned out that a slider in one of the front calipers was seized, the piston would push out onto the pads but the caliper would come out at an angle and bend the slider pin slightly. When pedal pressure was released the caliper would go back straight and push the piston back in so there was always a longer pedal travel.

To diagnose that, I clamped of the brake hose to each caliper individually and pressed the pedal to see if it was still soft or it would go hard, and then went around and did the next caliper. And released the last caliper etc so that I’m only clamping one brake at a time. I had a friend sat pressing the pedal. When I released the brake hose on the offending caliper I could see it flexing under the pressure which wasn’t obvious before.

Good luck


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[mid lid]

If the op has his foot down on the throttle on boost and then hits the brakes more than once it will defo run out of servo, what i mean is there may be nothing wrong if thats the situation he's trying to diagnose as you won't be doing that normally. Heel and toe is different as its very brief then immediately back to vacuum