Brake Pedal to Floor ... is this ABS / Brake Assist?

[eacmen]

I took my A4 to the track this weekend with race fluid, track pads, and SS brake lines - everything else was stock. During warm up lap (brakes are cool) my first hard brake the pedal would go to the floor and make a clunking noise - but braking did not seem to stop.

Is this just ABS with Brake Assist kicking into action?

Its raining and rather cool today, I have been able to re-create the same thing when the brakes are cold. Planning on replacing all the brakes this weekend though. Just wondering if this is normal for the braking system.

No warning lights or anything came on. I would assume if there was loss of pressure the brake system would go nuts and yell at me.

tia

[Ishikawa]

Who replaced your brake lines? It's possible some air got into the system somewhere. If you bleed your brakes, you can easily see when bubbles are present. I saw this the first time I flushed my system. I had to go through an extra container of new brake fluid just to get the bubbles out. Those pressure bleeding systems are a bit tricky to use without introducing air into your system. I learned that the hard way.

[eacmen]

I replaced the brake lines myself. I flushed a full liter of fluid through the system. No bubbles in sight. An audi mechanic on site also bled my brakes just as a precaution he also didn't see any air.

I use a Motive Power Bleeder to do the bleed at 15psi. Some people mention needing higher PSI to bleed the ABS system. Is 15psi sufficient?

[eacmen]

I use a Motive Power Bleeder to do the bleed at 15psi. Some people mention needing higher PSI to bleed the ABS system. Is 15psi sufficient?

I answered my own question. FSM says to bleed with 2 bar of pressure (2 bar is almost 30 psi). So 15 psi is not sufficient. This is likely what happened to my brakes, since they only are affected during a hard (ABS) stop.

Directly from Erwin FSM:

To bleed the ABS Hydraulic Unit -N55-, an initial pressure of 2 bar is necessary. Therefore, always check the pressure set‐ ting on the bleed unit.