Strange braking issue

[mbagge01]

I have a 2000 A6 2.7t audi that is having issues with what seems like the car is braking on its own when I let off the accelerator. It started happening around the time when my left front inner CV boot failed and the grease was all over the area of the wheel that I cleaned up when replacing the axle. I am not sure if the two are related but this is what I experience. When I first start the car up, braking seems fine but if my car is moving under extended periods of vacuum (idling or rolling down a road not under boost) the braking paddle starts to stiffen up and when it gets to this point, when I let off the gas, it is like I am braking and the car slows down on its own. If I get the car under a period of driving under consistent boost, to some degree it gets better and goes away for a while. I checked vagcom and there are no codes. Could I be dealing with something wrong with the master cylinder, ABS system or the brake booster. I checked the brake fluid level and it is fine. I also did the brake booster test by pumping the peddle with the car off and then holding the peddle down while start it up and the peddle will relax further, which my understanding is that the brake booster is working fine. I get the smell of the brake pads burning when I get out of the car because it seems like the car it fighting the braking system when this happens because I can also tell that acceleration becomes sluggish. One thing I did when it was really bad is when I had parked the car, I pulled the vacuum hose connection to the brake booster in the engine bay to relieve the pressure and then connected it back up to which the hard braking issues was first but only for a period of time.

[brokenwrench]

This issue would not be related to excess grease on the wheel or brake rotors. Does it feel like the brake pedal is being pulled down when it's acting up? Do your brake lights come on? It's possible one of the vacuum valves is malfunctioning an allowing too much vacuum to reach the booster. Have you tried to apply the brake while accelerating? The 2000 2.7t has the vacuum supply coming off the intake manifold (as opposed to a dedicated vacuum pump on 2001+). If the booster is indeed bad, you may not be able to apply the brakes at all when the manifold is pressurized. Maybe pull the vacuum supply line completely off the booster and cap it and see what happens (but certainly don't try this in an area where heavy braking might be required).

You certainly shouldn't drive the car more than necessary until you get this fixed, this sounds like a definite safety issue.

Good luck!

Edit: I just had another thought, have you adjusted the brake pedal screw lately? The vacuum valve in the booster should only open when the brake pedal is pushed in a certain amount, if the adjustment screw is too tight the valve may be kept open at all times. Maybe check for play in the adjustment screw to rule it out.

[mbagge01]

I read that a failing brake booster can present with intermittent issues of the brake pedal going hard tightening up the brakes like I am noticing due to intermittent vacuum problems before it fully fails. Does that make sense of a possibility. I do noticed that when I removed the brake booster hose, pressure gets relieved so I would assume that suggests that vacuum is being held to some degree.

The other thing I heard I should check is the check valves and the suction jet pump to make sure they are working properly.

[brokenwrench]

if the internal valve in the booster is failing, it absolutely could be causing your problems. I haven't priced a booster for these vehicles yet, but if you remove yours you should be able to take it to a competent mechanic to check out. Probably apply a vacuum to it when it's out of the car and see what happens.

You can also get a vacuum gauge and measure the vacuum pressure. I don't know what "normal" would be, but again a competent mechanic should be able to help you out with that.


Cheers!

[mbagge01]

My car was out in an ice storm for a couple days before some of these problems started. I have read about water getting into the booster. Any chance something like this may have happened when things melted. I did not see any water in plenum though.