gsimcox

I have a 2013 Q7 TDI and I am having a tough time diagnosing the brakes. The car has recently had an issue where the brake pedal goes to the floor without and resistance, you cannot even pump them to build pressure with the car on. If the car is off you will have brake pressure and the calipers will operate properly. I have bled the lines multiple times (the pump the brakes and hold and open bleeder screw method). If the car is off and you pump the brakes it has lots of pressure and will hold the pressure, as soon as the car is started the brake pressure goes away and pedal sinks to the floor. There are no leaks and I have not had to add any extra fluid. I recently replaced the brake pads and flushed the brake lines and replaced the master cylinder. I am at a loss as to what could be the issue, any ideas where to look? Is there something I am overlooking? Thanks for any help or advise.

'10Q7TDI_Prestige'

Well, you've done a 'complete' brake system overhaul it sounds like, so in theory it should be good, unless you are losing vacuum due to a broken fitting leading to the brake booster, etc.

Actually encountered much the same issue...pedal would just go to floor if trying to gently apply the brakes, and pumping them no longer brought the pedal back...just nothing. My issue ended up being the vacuum connection line from the engine that connects to the brake booster vacuum piping. The fitting is located on passenger side (USA/LH drive) engine bay just a couple inches from the firewall close to right front wheel well side. The connection point is down lower on the firewall; I repaired it from the top...just took my time doing it due to fragile nature of the plastic piping split. Basically, the plastic cracks over time from heat cycling of engine, and you lose some little bit of vacuum and the brakes just get weaker and weaker over time as the vacuum loss increases. As it was really very gradual, at least in my case, it was on my radar but not an 'immenent threat' to the car's drivability/safety...my brakes were worn, so I was thinking it was time to do the full brake job and flush the fluid out, etc. That wasn't it though.

Audi dealer wanted to 'replace all vacuum harnesses for about $1200, and refused to address my other concerns (warranty related) unless I allowed them to resolve the vacuum leak first. I went home and simply reinforced the bottom connector to brake booster line, reinserted the engine's vacuum line into the fitting, then custom-taped it up from the firewall opening up past the connection point to ensure it was an airtight seal; no leak. Brakes worked as normal again immediately; brake dust and all. The loss of vacuum was also triggering some other coolant temp error codes in my car, so those active codes went away too once normal vacuum was restored.

gsimcox

This is exactly what is happening. It was gradual. I'll look for the the vacuum line and see if I can find it. This thing has taken up way too much of time.

gsimcox

Do you have a picture of where this is located? I cannot seem to find this fitting.

bg_2007Q7

That's a scary problem. I hope you can find the solution.

I had a GTI long ago, that lost brake pressure on the interstate. Thankfully, it was a 6 speed and I was able to go onto the shoulder and downshift.

'10Q7TDI_Prestige'

So my prior description was for the Gen 1 TDI 3.0 engine, and I got to looking at our '15 (Gen 2 TDI), and the booster's vacuum line comes out through the firewall in same place, close to the coolant reservoir, but further down on the actual firewall (separates engine compartment from the plenum/under-windshield area, etc.), but on the Gen 2 TDI, the booster hose connection angles off to right, as viewed from the front of engine vs. pointing straight up like it does on the Gen 1 TDI. The connector type used is diff on our '15 too...less gimmicky, and so should be an easy fix if that connection is leaking vacuum.

Definitely check that as it is 'low-hanging fruit' and an easy fix. You may have other vacuum issue resulting from where you did work on the master cylinder, which connects to the brake booster, etc., and there are vacuum connections/hoses there also, that will create same symptoms if leaking vacuum.

Good luck!

'10Q7TDI_Prestige'

Hey, thinking of you; just ran across a video of guy who replaced his master cylinder, flushed the brakes, and has kinda same issue you're having now. He used a pressure bottle bleeder initially and thought everything went well, but actually still had air trapped in the system.

Seems like the problem is when you crack open the master cylinder, the brake system then needs to be completely bled, not just the standard 'flush' where you pump through each brake circuit/line, etc. He went back and used a VAGCOM (in his case the OBD-Eleven) to activate the ABS pump while he was flushing the brakes again (he used a pressure bottle too, etc.). He really didn't see any air bubbles until he got to the last wheel, and then there was just a ton of air bubbles coming out of it.

Anyhow, his isn't a Q7, so the only thing the video may offer is simply his setup to ensure battery voltage to run the ABS pump, where to access the ABS pump in VAGCOM, etc. Remember to bleed both side of each caliper starting with the inboard bleeder, etc. There's likely other vids or threads that discuss running the ABS pump to do a full-system bleed to ensure all air is out of the system. Use of the pressure bottle system does help to ensure you don't introduce air back into the system while bleeding it, and takes some variables off the table for errors to be made:

gsimcox

I did find some cracks on the vacuum pipe and used epoxy to seal them up. That seemed to help a little. I did the ABS bleed process with VCDS. I got lots of air out that way but I also managed to break both front inner bleeder screws. I believe lots of air trapped in the inner side though. I can at least pump the brakes when it is running but I need to figure out how to replace the inner bleeder screws to finish the job.

'10Q7TDI_Prestige'

Yeah, getting cracks in/around the junction point where the engine vacuum line meets the brake booster line just outside the firewall is an extremely common issue, but it sneaks up on you over time.

The more I look around, there's a ton of info/videos about needing to cycle the ABS pump on modern vehicles to ensure all trapped air is removed during a brake flush/bleed of system after doing work on it or just having a spongy/low engaging pedal, etc. Now, if you find the pedal with engine off is rock hard and no movement after a few pumps, you are likely 'good'. On mine, I've got an inch + or so of spongy travel prior to hitting that 'rock solid' wall, so I'll be cycling my ABS pump in VCDS this weekend!

It's interesting that in VCDS it has a programmed 'basic' function to bleed the brakes via the ABS pump; it makes perfect sense now.

-Wes-

The bleeder screws should be a standard part that most auto parts stores will have (not "audi", but should be able to match if you bring an old one in). If the bleeders are stripped, then just use some vice grips to lock on and turn it out. If it is actually broken off in the caliper housing, then try a blade-style stud removal tool, or even a drill bit, to lock on and back it out.