Hi crew,

I have a bizarre issue with my 14 Q7. Had an ESP light which turned out to just be a wiring issue between N247 brake magnetic coil wire #6 and wire 31 at the ABS controller) and instead of doing the original proper electrical troubleshooting, I went whole hog and replaced the booster, master cylinder and the ABS pump.

Now, I'm trying to bleed the system and I am getting no fluid out of 3 of 4 wheels. Passenger rear is the only line passing fluid oddly.

I have 28+ in Hg vacuum at the booster and the vacuum is holding (no leaks). Pedal is extremely firm and at the top of travel..it does sink slightly on engine on, so I "think" the booster is ok. Also on ODIS ABS purge routine, pedal does go down, but still no fluid movement at the calipers.

I opened the brake lines at the master cylinder and fluid is coming out. I'm using a pressure bleeder at 30psi.

I opened the valves at the ABS block and I'm only getting fluid out of 4 of 6 lines. The two lines that aren't passing fluid at the ABS are the lines labeled DK (I think this is also called the pressure circuit, but don't know where that hard line goes) and the VL (drivers side front caliper I think). VR, HR, HL, and SK are all passing fluid.

ABS block has power and I can command it to purge valves..But on the Output test for hydraulic valve the pedal sinks which according to ODIS means a failed ABS hydraulic valve, but I've now replaced that component with two different used ABS pump units and get the same issue.

Everything is coded properly, and there are no fault lights in ODIS for any component.

How can I further troubleshoot this. Originally I thought it was air in the system, but now I'm feeling like that's unlikely. Is it really possible that I have a bad booster? I'm not hearing any pressure leaks Beyond pressure bleeding the master and loosening the fittings, allowing any air out (I'm getting all fluid there) are there any other tests I can do to verify booster or master cylinder issues?

Thanks for any help here. I'm perplexed.

 

That's funky you not getting any brake fluid through to pass on three of the wheel calipers.

My bleeder valves are getting gunked up and nasty/difficult to seal up, etc., and can be quite temperamental.

I would suggest a process of elimination.

First, though it may sound off-base, I would 100% verify you got a strong seal at the vacuum pipe/hose connect between the brake booster and the engine-side vacuum. That connect is right inside back of engine compartment / passenger side / firewall where the hose line from the booster comes through firewall, etc. Reason why: That is a very common vacuum leak location, and a vacuum leak will cause wonky things to happen with these brakes and other vacuum driven systems on this car. Been there, done that...fixed my vacuum leak and brakes functioned normally again.

Second, the only successful method I've used on our Q7s is to run the ABS pump using VCDS or similar scan tool to trigger the 'basic settings/functions' in that control module, which then walks you through a process to go through all four wheels in sequence, and it runs the ABS pump as you go through the sequence to push all the air out of the system, etc. Since you replaced ALL the hydraulic control parts, that would really be the only method to get the brake fluid in and purge all the air out. There is a limit on the PSI you should use with the pressure bleeder hooked up, but I don't recall it off top of my head. I want to say my pressure bleeder and/or the Audi repair reference said don't exceed 20 psi, as you can damage the seals in the master cylinder and other bits too, but I don't recall exactly. I did own a car prior where paid the dealer to do a brake flush and they used too high a pressure and blew out the master cylinder and also a caliper seal(s), resulting in 'no brakes' scenario, and dealer had to replace both.

I no expert on the ABS pump, but a lot of stuff on this car simply comes down to having the steps from the factory repair manual to follow, and refilling/bleeding brakes/swapping out master cylinder and ABS Pump are no exceptions.

Remember there are two bleeder nipples on each caliper (inner/outer), and a defined bleeding sequence to follow. Might have some garbage got stuck in the lines somewhere, as the ABS pump won't keep you from bleeding the brake lines in the 'classic' manner, it's just not complete a 'complete air bleed' unless you actually run the ABS pump during the bleeding sequence as defined in the repair manual. I'd likely try removing the bleed nipple entirely on one side of a caliper if not getting any fluid to flow through it just to ensure it's not corroded internally or blocked, and if it then flows brake fluid you know what the problem is. If not, then the actual issue is upstream.

Good luck!