Brake pedal sinking

Hey guys! I picked up an 03 jetta about 2 months ago to replace my current daily driver. This is my first VW, but I am not totally new to working on these things since my friend has had one for the past 7 years or so. Anyway, lets get down to it. I have been slowly working on this car to get it through inspection and have run into a problem.

With the engine off, my brake pedal seems fine, its firm and will hold pressure. After starting the engine, the pedal immediately sink to the floor. I did not have this problem when I purchased the vehicle, so it must be something I created. I have done insane amounts of bleeding, both the old school way, and using VCDS to purge the abs unit. I have gotten a lot of air out of the brakes, but there has been no difference in pedal feel. I have ran almost 2 liters of fluid through this thing, so I really don't think there is any air trapped in the system. I am wondering if I just got a bad master cylinder? Any help would be greatly appreciated!

"Parts" I've replaced

-flushed the brake fluid (motive bleeder)
-replaced 2 calipers ( on because the bleeder screw snapped off, the other was making odd clicking noises when I would push on the brake)
-replaced the master cylinder

Are the bleeder screws pointing up on the calipers?

Hey, I had that exact same problem after replacing my rear calipers as well. When I bought my car, the owner had been driving around with a brake fluid leak and had run the brake fluid reservoir dry I assume. Even after bleeding all 4 corners over and over, the sinking pedal problem didn't get any better. Turns out you have to bleed the ABS system with VCDS, then all 4 corners. After that, my brakes felt better than ever.

My son's '02 has had a soft brake pedal for a while. Replaced brakes (it was due anyway) bled them twice. One thing I learned is that the Motive bleeder doesn't build enough pressure to get the air out of the rear calipers. Pumping the pedal once or twice and holding it down while tightening the bleeders helped. We did this with the motive attached and pumped up. That helped.

Replacing vacuum lines also helped. Sometimes the soft pedal is a vacuum leak, not hydraulic.

Enabled said:

Are the bleeder screws pointing up on the calipers?

Click to expand...

Yes they are, I never even thought about that.

So you guys think its still air in the system? I used the vcds to bleed the system several times....probably 5 or 6 cycles at least, but I wasn't getting any air at the bleeders after that so I figured the system had to have been fully bled.

Yea, in the past I was asked by some locals to help them bleed their BMW after caliper rebuild, with the diagnostic programs. After 8 (!) Liters, there was still air and sinking pedal... Even with abs activation.

Upon further inspection we found that they inversed the calipers, and bleed screws were pointing downwards.