My A6 had hard brakes after sitting for a year, determined it was a bad master cylinder and/or booster.

So I replaced the Master and Booster and now the pedal goes right to the floor. Does this mean the new booster is bad or that I have a large amount of air in the system from replacing the master and booster?

 

Did you bleed the system after installing the new master cylinder? If you did not then, yes, you have air in your system. Any amount of air is bad.

 

Have not bled it yet since MC change (I did just before when I changed the front brakes in an effort to make it work).

One contributing piece of evidence is that my brake electric vacuum pump (2001 A6) runs continuously after the MC swap. That is why I thought maybe this new booster has a leak? Is that assumption flawed?

 

You absolutely have to bleed it after a master cylinder swap. There is no way to do that without getting air in the lines and it does not take much air to make the pedal mushy.

I did not realize that any gasoline powered audi's had an electric vacuum pump. If it is running all of the time then you have a leak somewhere, perhaps the booster or perhaps a hose. But a leaky booster will not make the pedal go to the floor. It can make the pedal stiff like the problem you were having before.

 

Ahhh, now I am confused.

Previous to MC and Booster swap: Hard hard hard pedal and brakes barely slow vehicle. Could hold pedal down and it would not move. Electric vacuum assist turned on rarely.

* Checked all check valves and vacuum lines. I want to say it was holding 18 for vacuum (whatever the value it was matched the spec in the Bentley for 2.8 idle)

After MC and Booster swap: Pedal has hardly any resistance. Does not pump up, no change with engine off. Electric vacuum assist runs endlessly

* I checked and the push-rod/ball on booster is seated correctly in the brake pedal assembly
* All vacuum lines are hooked up.
* Plenty of brake fluid in reservoir

I could bench bleed the MC from what I am reading and do the normal bleed routine but it sounds like you are telling me that will not help the current situation? Thanks for the help, this is the last thing to get this car back on the road

 

I went ahead and bled the MC and the brakes firmed up... and the vacuum pump keeps on running still. Sounds like MC is good, and booster is bad?

Is there a way to test a booster with it off the car? I have a manual vacuum pump/gauge (sears model).

 

It does sound like the booster is bad. You should be able to test it in the car. Just remove the vacuum hose and connect it to your pump. Pump it down without your foot on the brake. It should hold vacuum. I suspect that it will not since your pump never stops. If it does hold vacuum do the same thing with someone pressing on the brake pedal. The vacuum will drop when the pedal is first pressed but then it should hold.

If the booster holds vacuum then you have a bad vacuum connection somewhere.