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Hey All.

My daughter got her license earlier this week so I'm getting my old passat ready for her. (2004 V6 4motion) Yesterday was 4 new tires and today was going to be new pads.
Been doing my own brakes for 30 years and never ran into this. (first time doing a VW though)

The brakes were fine and had plenty of pressure before starting the job. The brake indicator came on a while back so I thought i'd replace the pads. We started with the left front. Pulled the old pads, installed the new pads, connected sensor wire and put the wheel back on. We lowered the jack and I took a seat and pressed the brake pedal. Everything felt fine (good pedal / pressure) However, I then stared the car and the pedal went to the floor. I pumped the brakes and there was a little pressure, but after a few seconds of just sitting the pedal would then easily go to the floor.

Not sure what was going on I decided to change the right front... removed the old pads, installed the new pads, connected sensor wires etc.. Put wheel back on.
I got in the car and pedal pressure was fine with the car not running, but once I started the car the pedal lost all pressure.

I used a c-clamp to push the piston back in before installing the new pads.

After doing the right front I noticed a rather large, roundish wet spot on the driveway. Maybe 10 inches wide located about directly below the brake fluid resevoir.. maybe a bit behind that.

Not sure what to do we decided to bleed the lines. We did RR, LR, RF, LF. My father was operating the pedal while I was opening and closing the nipple. Progress seemed to be being made as he was saying the pedal was feeling good. After doing the last line (left front) we packed everything up and I got in the car, started it, and the pedal went to the floor.

We did bleed with the motor running. i believe I may have read before writing this that bleeding with the motor running is not a good idea? True?

What we found odd is the fact that the pedal felt fine with no motor running but would immediately lose pressure once the motor was started. We did NOT put a block or 2x4 under the pedal when we bled the lines.

As i couldn't leave the car sitting in my father's driveway I nursed it back to my house 3 miles away.. it stops, but I would never trust it under normal driving onditions. Minor pumping would give a little pressure, but, as mentioned above, after a few seconds of not pumping the brake the pedal would once again lose pressure.

I've been reading about "bleeding the ABS" (or something like that) could that be part of the problem. I do not have vagcom but do have a regular error code reader.

Any suggestions would be very much appreciated.

Dave
 

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At any point during the bleeding, did you run the brake fluid reservoir low? If so, that may have introduced air into the system.

When you pushed the piston back into the caliper (to fit the new pads), the displaced fluid backed up into reservoir and overflowed (that's the spot on the floor under the reservoir).

I created a simple pressure bleeding adapter that I use with about 15psi of air from my compressor. In this way, I only have to crack open the bleeder screw at the caliper and let the fluid flow through. The only thing is to make sure the reservoir stays topped off - this is done by checking the level and topping it off between each corner (RR - LR - RF - LF - clutch).
 

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That's the situation; air entered the system. Maybe not much, but enough that it can be compressed by one stroke of the master cylinder. In the future, don't force the old fluid in the caliper back up the system. Attach a clear tubing to the nipple, then open it while forcing the piston back. Another advantage is that fluid won't overflow, and might not even need to be topped up after the new pads are installed.

No benefit from running the engine, but running the ABS pump might be necessary, unless you use a pressure bleeder such as khnitz suggested. A code reader won't help with the ABS pump; a licensed copy of VAG-COM will.
 

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This is why I invested in a hand-pumped pressure bleeder. I use mine for air pressure only (I don't fill it w/ brake fluid). Works like a champ.
 

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If air got in the system, I don't understand how it got there by just changing the brake pads. Is there a hole somewhere?
 

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Typically that happens when the reservoir is so empty that air is drawn in rather that brake fluid. But I agree that isn't consistent with the O.P.'s description.
 

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Guessing your fluid on the driveway was from brake fluid overflowing the reservoir when you pushed the pistons back. You should see some sign of this under the master cylinder (wetness and possibly paint eaten away by brake fluid).

You have to assume air in the brake lines first, so they need a proper bleed. If that doesn’t solve it, then master cylinder. Potentially a MC piston seal was damaged when fluid went back up - sounds unlikely but maybe?
 
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