BIG issue after Brake Master Cylinder replacement

[CO AVANT]

So I installed a new brake master cylinder without a hitch (ATE brand just like stock). Bled brakes and slave cylinder twice. Start the car and brake pedal goes to the floor. The rod that goes through the brake booster is in the master cylinder piston so that's not the problem. I even installed my old master cylinder and re-bled. Same issue. Brake pedal goes down to the floor. I did exactly per Bentley manual and now have zero brake pedal force. Pedal goes to the floor with hardly any effort so the brake booster vac is still working properly. I can get pedal force with the car off if I pump on the brakes, but the moment I turn the car on the braking force is gone and the pedal goes down to the floor. No leaks, all lines are tight. The only test for abs pump I can do is the output test in vag with cycles the pump (you can hear it when it turns on). So why the hell am I not getting any brake pedal force anymore? I know it's not the MC causing this. Only thing that comes to mind is the hydraulic control unit or ABS controller/pump... if it was a booster issue the pedal would be hard as a rock, not easily dropping to the floor. anyone have any idea??? This is driving me nucking futs

[Papachristou]

damn, i was hoping you would have some insight to our problem this week! maybe old guy or diag will chime in!

[dougyfresh]

Is there any way to test the ABS unit?

[CO AVANT]

I can output test it and hear it kick on and run....so that leaves me to believe it's functioning properly...

[AudiA4_20T]

Jordan was this your problem before also?

[CO AVANT]

No not like this...

[Papachristou]

our problem is just lack of initial and good grab... with the ferodo pads, my car grabs much more once they sink in but still not much initial grab, i hear other b6s have much more

[dirtybrd]

Have asked your other Audi buddies/techs?

[CO AVANT]

not yet, both at work and cannot reach them. Since I didn't take the pedal off and moved the brake booster around to get the MC out I'm thinking I might not have the push rod from the pedal to booster positioned correctly.....even though I could have sworn it was positioned correctly... maybe that is why the Bentley manual says to remove the rod from the brake pedal before replacing the MC? The MC hits the firewall so the only way to get it out without removing the rod from the brake pedal is to take off the wiper blades and crowl. Gives you the extra room needed to twist the booster and get the MC out...

[102487thedude]

do to your climate and contions you installed the fluid in you may need to go through a lot of fluid becuase it sounds like you have a big air pocket in the brake lines somewhere.

the only way to get that air pocket out is to push the fluid in front of it out of the lines which will be wasted but it sounds like that is what needs to be done.

[102487thedude]

also it is really a two person job becuase you need some one buy each caliper to release the valve while you compress the pedal and then they need to close the valve before you release the pedal it could take an hour or so tocomplete.

[CO AVANT]

I'm using a motive power bleeder and have sent more than 4 bottles of fluid through the system. I get brake pedal firmness with the car off but it goes to mush when I turn the car on. I think the air is out of the system unless there is air that can't escape in the hydraulic controller unit since the lines from the reservior to that unit were disconnected for the MC replacement.

Does anyones vag 03 ABS - Basic settings - group 002 actually turn on/off to bleed the ABS system? That is what ross tech says to do to bleed the ABS system but neither groups 001 or 002 do shit on my vag-com...

[AudiA4_20T]

I'm using a motive power bleeder and have sent more than 4 bottles of fluid through the system. I get brake pedal firmness with the car off but it goes to mush when I turn the car on. I think the air is out of the system unless there is air that can't escape in the hydraulic controller unit since the lines from the reservior to that unit were disconnected for the MC replacement.

Does anyones vag 03 ABS - Basic settings - group 002 actually turn on/off to bleed the ABS system? That is what ross tech says to do to bleed the ABS system but neither groups 001 or 002 do shit on my vag-com...

side question, but what happens when the car turns on? Why does the pedal compress like it does?

[317ssayzarc]

that circuit can be a real whore to bleed, that one and the slave... try jacking up the front of the car and bleeding it some more, trust me and just try it

[CO AVANT]

I think I have air still trapped somewhere between the MC and ABS/hydro pump. I pumped the bleeder up to 35psi and now have tons of bubbles coming out the calipers. I mean tons. I have bled 3 containers full through the motive bleeder and still have bubbles. John (Diag) told me I could run 55gal through and still not get all the air out if it's trapped in those locations. He said to try cracking the line nuts at the MC, hydro, abs while the bleeder is pressurized and try to bleed from those locations. Guess I'll try that today... Jake wouldn't you want the front end lower than the rear if the air was trapped in those locations? since that is the highest part of the system you'd think you'd want to lower it so the bubbles go through the lines and out the calipers easier

[dougyfresh]

Jake wouldn't you want the front end lower than the rear if the air was trapped in those locations? since that is the highest part of the system you'd think you'd want to lower it so the bubbles go through the lines and out the calipers easier

It wouldn't matter. You've got the system under pressure. If all you had was ambient pressure then it would matter (like siphoning fuel from a gas tank).

[CO AVANT]

ahh, gotcha... I'm going to try bleeding at the MC/Pump/ABS and see how it goes. My bet is that I'm gonna make a mess...

[dougyfresh]

Best of luck to you. I'm sorry to hear you have to go through this schit.

[absolutegtr]

Back when I was in automotive classes, I worked on a car that had similiar brake problems to what you having. What I ended up doing was exactly what John, (Diag) told you to do.

I had to first bleed the ABS pump, before continuing on to the rest of the brake system......Its like we had to bleed the system in "sections".

Futhermore, maybe try bleeding the system with the car running.

[102487thedude]

when bleeding the lines the trickis to open the nut for the air to escape on compression and before releasing the pressure close the nut on the caliper fast. if this is not done you will just keep sucking in air through the opening during the relase.

the reason for the pedal going spft is becuase the brake booster is stronger than you and is ompressing those air pockets


remember squeez pedal down open nut fast than close nut fast than release pedal. make sure you do not realess while nut is open.

i would not loosen the line in the middle becuase you will give your self a bigger problem than you already have.

[dirtybrd]

This is the reason I suggested bleeding just the master. There is no reason to push air thru the system if you don't have to. Anytime I replace a master, I bleed it right at the lines.