[Early_S_Man]

This is not a decision you can make without disassembling the master cylinder and inspecting the bore ... generally pitting will be visible on the lower crescent of the bore if absorbed water has damaged the master cylinder!

If there isn't any pitting on the lower arc of the bore that you can feel with your fingertip after running a brake cylinder hone in the bore ... then you can be confident that a rebuild will be successful, assuming that you assemble the new parts correctly! All new parts ... pistons, cups, and seals are provided in the ATe rebuild kits. I have never had a failure within five years on any master cylinder I rebuilt in the past 30 years+ ... and I have been using Dow Corning 4 Silicone Grease to lubricate the bore, cups, and seals all that time! Ate used to provide a tiny tube of identical silicone grease with all of their 911 master cylinder kits, though not in VW kits!

1. Use isopropyl or methyl alcohol to clean the master cylinder body and bore ... especially after using a brake cylinder hone!

2. Use a genuine ATE rebuild kit.

3. Use silicone grease to coat the bore of the used master cylinder after thoroughly drying the bore after cleaning with alcohol. Use silicone grease on all of the cups and seals prior to installation onto the pistons.