[Vwdiesel] Brake Fluid, '91 Jetta

[William J Toensing]

Can someone out there tell me what the difference is between Dot 3 & Dot 4 brake fluid I always thought Dot 4 was a higher quality brake fluid that was compatible with Dot 3 & was for use in heavy duty or more severe operating conditions. Dot 5 is silicone brake fluid. I like it because it is non-hydroscopic, that is it will prevent rust in a brake system. I am using Dot 5 in many of my collector cars. I rebuilt the brake system in my '41 Ford over 20 years ago & it still is working fine. However a mechanic friend of mine doesn't like it. I understand VW recommends against Dot 5. Does anyone know why? I have not put any Dot 5 fluid in any of my VWs or my modern cars. My understanding is for Dot 5 to work fine & prevent rust you have to use all new rubber components in order to prevent rust since Dot 3 & 4 will absorbed into the rubber components  & eventually absorbed water, a concern with collector cars that sit idle most of the time.
Bill Toensing, Nevada City, CA