Just a few words on bleeding conventional brake systems. I think this ist absolutely and completely true for the Audi, as well.
" Bleeding brakes is not one of my personal favorite jobs. There seems to be a bit of black magic involved with the bleeding process. Sometimes it will work perfectly, and then other times it seems like you end up with a lot of air in your system. The best strategy to follow when bleeding your brakes is to repeat the procedure several times in order to make sure that you have removed all the trapped air from the system.
The right tools are a necessary part of the job too. A few days before this book was to be sent off to my editor, I had a chance to evaluate a new type of pressure brake bleeder kit from Motive Products. Retailing for about $45, this kit attaches to the top of the master cylinder reservoir and applies pressurized air to the system. Brake fluid is forced out of the master cylinder reservoir and into the system. The pressurized kit is probably the best one around because it is the least likely to create air bubbles in the system. There was a time when no one was manufacturing pressurized bleeders, but thankfully Motive Products now supplies this excellent quality kit at a reasonable cost.
The pressurized system works very well because it pushes the brake fluid out of the reservoir and into the system. In this manner, it is very unlikely to create air bubbles in the system. When small air bubbles form in the brake lines, the entire system suffers as the brake pedal becomes soft. This is because air is much more compressible than the brake fluid. When you push on the pedal, the air trapped in the lines acts like a spring inside of the system. The air becomes compressed, absorbing energy from the system, instead of directing the energy towards pushing the caliper piston against the brake disc.
A second alternative is the vacuum bleeding kit. This kit works in the opposite manner of the pressure bleeder, applying a vacuum to the brake system in order to draw brake fluid out of the car. The system works well, but can sometimes cause air bubbles to form in the lines. Particularly on cars with rear brake proportioning valves like the Porsche 914, the vacuum system can leave air trapped in these valves, giving a spongy pedal as a result. When using the vacuum bleeding system, the best approach is to bleed each corner of the car several times, in order assure that all the air is out of the system. Simply fill up your brake reservoir, attach the pump system, pump up some vacuum, and then open the bleed nipple. Brake fluid should be pulled out of the system when the vacuum is applied. If it’s not – you may have a problem with your brake lines.
The third and most labor intensive method of bleeding your brakes involves actually having an assistant press on the pedal while you go around to each wheel and bleed the system. Without a doubt, this is the most effective method of bleeding, and should probably be used as a final procedure when performing any brake system bleeding. This method actually pushes fluid through the system (similar to the pressure-fed system) at a high rate of velocity. Sometimes, air bubbles that are in the system can become dislodged and cleared out by the quick rush of brake fluid when you press on the brake pedal.
The procedure for bleeding the brakes using the brake pedal is pretty straightforward. Attach a small rubber hose to the brake caliper nipple and let the other end hang inside an empty container. Ask your assistant to firmly and quickly press on the pedal 3 times, and hold it down the third time. Then, open up the bleed nipple by unscrewing it slightly. Brake fluid should come rushing out and the pedal should sink to the floor. Make sure that your assistant doesn’t remove his or her foot from the pedal, as that will suck air back into the system. With the pedal still depressed to the floor, tighten up the bleed nipple. When the nipple is closed, have your assistant remove their foot from the pedal.
I recommend that you use this procedure as a final step, even if you are vacuum or pressure bleeding. The high force associated with the pressure from the brake pedal can help free air and debris in the lines. If the brake fluid doesn’t exit the nipple quickly, then you might have a clog in your lines. Brake fluid that simply oozes out of the lines slowly is a clear indication that your rubber lines might be clogged and constricted. Don’t ignore these warning signs – check out the brake lines while you are working in this area."
http://www.drivewerks.com/tech/mult_bleed_brakes2.htm
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