bavareze

Hello,

I have 99 Audi A6 with V6 ATQ engine. I have a brake issue:

after driving normally (accelerator 50% => low vacuum in manifold) the brake pedal is very hard = no boost.

if I let accelerator pedal go for few seconds ( = lots of vacuum in manifold) then the brake pedal is again normal.

what I am guessing is that a certain valve that is supposed to trap the vacuum in the booster area is not working properly. so whenever accelerator is open the vacuum is lost.

any idea which valve might be bad? or any other ideas?

thanks

jseklund

Did you check your lines off the brake booster and to the intake, etc??? I seem to recall someone else having some issues with one of the hoses popping off the where it connects to the back of the engine. When the vaccum would build, it would suck the hose shut, but under enough pressure - it would break the seal and cause issues.

Just a guess...but a place to start.

bavareze

I found two check valves located as you see in the picture. One of them was stucked open. So I cleaned it and now it seems to be braking much better.

cdmx

I realize this is an old thread, but when searching google this shows up
I had the stiff brake pedal as well and it took some thinking and some help from you all and google searching. BUT no one had a clear answer.
I used a vacuum gauge at one of the vacuum line T's towards the front of the motor and noticed a drop in pressure which was the cause of the hard brake pedal. I checked all the areas other people mentioned but found the culprit to be the vacuum pump connection line o-ring. It was causing a loss of vacuum, I could press down on the hose attached to the vacuum pump fitting and hear an audible difference and a rise in manifold pressure. So I hope this reply helps some other people in diagnosing their problem. (btw they wouldn't sell me the o-ring or fitting seperatly I had to buy the pump complete, after I had taken the old one apart I believe I could have matched up the o-ring!)

SloopJohnB@mac.com

More likely a cracked vacuum reservoir. Find it. Check it. Replace as necessary.
Along the way you can check all the lines, etc.

ksebby

I'm having a very similar issue. I don't seem to have enough vacuum to the booster. When I start the car with my foot on the brake, the pedal goes down a little, but not nearly as much as it should. Pedal is hard and don't have much stopping power. The valve at the booster seems to be working properly--holds vacuum. Can't find any vacuum leaks by listening and spraying carb cleaner around. When driving it is pretty hit or miss whether I have brake assist or not. I get good assist if I am driving reasonably fast and push on the pedal lightly, but if I push down harder I loose most of my braking power. How much vacuum should there be at the booster? I pulled the vacuum line and valve out of the booster and stuck my finger over it; it held onto my finger but didn't pull THAT hard. Does anybody have any insights into this? Thanks.

ksebby

OK. I think I'm having trouble with the valve that is shown in the bottom picture as well (vacuum control valve). I can blow through it in every direction. Doesn't seem appropriate for a valve...

SloopJohnB@mac.com

Get the p/n and replace it. It's a check valve.

ksebby

Wasn't the valve. I replaced it and the problem wasn't fixed. I also took apart the original one which has two little rubber flaps inside which all seemed in good condition. Apparently, you can blow through this valve in every direction when it is in working order--I could with the old one and the new. I have checked my vacuum lines from the engine to the booster and everything looks good. I also replaced all the hose clamps just for fun. The only place I can see that might have a problem is the booster itself; it doesn't make sense to me and I hate swapping out parts without being sure of what the problem is but that's my next step unless anybody had another idea...

N_Jay

Pull vacuum on booster from an external hand pump with gauge.
See if it leaks down.

If it does it is bad.