Background: I have a 1998 A4 coupe that is stock with 185,000 miles. I need to replace my brake master cylinder because the resevoir is haso cracking and I want to replace it before it starts to leak. It looks like it is easy to replace by removing the two bolts and attaching the break lines to the new m/c. Are there any gaskets I need? Do I have to worry about air getting into the ABS. How do I bleed the master cylinder?
I did a search, but just got a bunch of threads on how to change pads and rotors.

I'd appreciate any help.

 

Here's how I replaced the master cylinder. Before starting be sure the car is up on stands with the wheels removed to bleed the brakes. First step, bench bleed the new master. I did this by attaching fittings to brake outlets and the run tubing from the fittings into reservoir. Fill the reservoir with fluid and actuate the piston. Most attach it to a vise. I ended up using a small gear puller to actuate it. With the puller I used, I put a small socket on the piston to keep the middle piece centered. Which kept it from moving around on the piston. You can use whatever method to actuate the piston you choose, this is just what worked for me. Continue actuating the piston until you have no air bubbles coming out of the lines. I did it for quite a while after I quit seeing bubbles. Also make sure your tubing is short enough that the air wants to travel upwards.

Now back at the car, put thick towel (or two) on the fender. Brake fluid will remove paint. I also would suggest a towel under the master to catch any fluid. Take a turkey baster or syringe and remove all fluid from the master cylinder. Remove the level sensor clip. Now break loose the 2 lines, but don't remove them or loosen them to the point the fluid is running out. Remove the two nuts that secure the master cylinder to the vacuum booster. If you can locate something to cap the lines do so. Remove the lines and remove the master existing master. Cap the lines if you can. Back at the bench remove the bleed fittings/tubing and quickly set the new master on the vacuum booster studs (used my fingers to plug the line holes while transporting it). Tighten the lines up as quickly as possible, then replace the nuts. If you removed the brake vacuum line, don't forget to replace it. Attach the level sensor harness. Bleed the system normally.