Gasoline and Dream

Hi, my friend have an Audi Q7 3.0 which has a total brake loss problem.

At first, we changed all of the front and rear brake pads and within 7 days the brake got totally loss without any indication.

So we tried to bleed the brakes, we had make sure that it was already normal after the bleeding. There was no air and bubbles. And one day after the bleeding, when we test drive it the brake got totally loss without reaching 5km distance-long and the ABS indicator shows up in the cluster while the brake got totally loss.

So we scanned, and the scan result showing us the rear right ABS Sensor.

My Question is:
Does the ABS sensor Cause this problem? Or is there any other thing that Cause it? How do we fix it?

​​​​​Thank you

​​​

-Wes-

ABS sensor isn't going to result in no brakes, just no anti-lock brakes. If you have total brake loss, then you either have a bad booster or some vacuum leak related to its function, master cylinder, or a line blew out somewhere due to cracking (soft line) or rust (hard line). Was the fluid ever low when doing any of this work? If not, that would rule out a physical break. It's also possible to have an internal clog in an orifice somewhere if your friend has been neglectful with regard to fluid replacement. All brake fluid should be flushed every 2-3 years depending on use and conditions.

We have several brake-focused threads here. Read through them.

'10Q7TDI_Prestige'

So, if the brake pads weren't already worn out prior, then changing them has no impact on resolving the issue. If the brake pedal is 'hard' that means there is no vacuum assist from the brake booster, and it will feel like there is no braking happening...the pedal will remain high and you can't depress it. If the pedal goes to the floor in car, that's likely the vacuum leak impacting the brake booster. Likewise, there is generally always air trapped inside the ABS unit, and the only way to get it out is by bleeding the system using a scan tool to run the ABS pump as part of the bleeding process. Good threads on here on this topic.

Another common, underlying cause is the vacuum line connection inside the engine bay that links to the brake booster's vacuum line at the firewall...well, that little plastic connection (on the brake booster's vacuum line side) gets brittle over time and literally cracks/peels apart like a flower, thus creating a vacuum leak that gets worse over time. When it is a large enough vacuum loss, then the brake booster stops functioning. If the brake booster goes bad internally a similar condition of no boost will be present at all times, but check the vacuum line connection first as that is a common issue. There are very simple tests to verify if the brake booster is bad if you can verify the vacuum line connections are not leaking.

Gasoline and Dream

Yesterday, we had already repeat the bleeding processing. And the brakes turns normal again. And this morning when we test drive the car again, the brake got totally loss again without any indication. But we heard a weird sound and we suspect that the sound was coming from the engine side. And when that sounds stop, the brake got totally loss and the ABS shows up on the cluster. And the scan shows the same problem, which is the rear-right ABS sensor. It's so confusing.

'10Q7TDI_Prestige'

This is a confusing car, and also a difficult to repair car if you don't have the correct type scan tool; a VAGCOM scan tool. If you use the wrong type of scan tool (an old-school OBDII), then you can only see trouble codes linked to the CEL/MIL indicator lamps on the dash display, etc., so you don't see the whole picture of what is going on. A VAGCOM reads the CANBUS linked control modules (programmed in VW coding) on this car, so that's why you need a VAGCOM capable scan tool.

The 'weird sound' you failed to describe is likely the key to you locating the underlying issue. Vacuum system leaks or part failures (PCV) that impact the crankcase vacuum levels can result in odd noise/noises from the engine bay. If confident you've done the brake fluid service properly to remove any air from the lines and the ABS pump, then move on to locate/localize the problem. Also suggest you do the static test on the brake booster to ensure the internal diaphragm inside it hasn't failed; look it up. FWIW, the ABS pump is in the engine bay region, and it can make some funky, loud noises when it runs, but it is also very mechanical sounding when it cycles and consistent, and you can feel the vibrations from it in the brake pedal, etc.

The more details you can provide (pedal is hard during braking attempt/no brake assist felt, or the pedal goes straight to the floor...pumping the pedal has no impact at all and there is a complete loss of braking effect as pedal goes to the floor, which generally indicates a bad master cylinder. Know your car's specific symptoms and then it'll be easy to compare them to brake system diagnostics available online

Gasoline and Dream

Yes, The Pedal goes straight to the floor and pumping the pedal has no impact at all. So it's the Master Cylinder which cause the problem?