From Andover's monthly newsletter: Brake Fluids: A warning from Ashley This is what happens when you use a ‘’DOT 4 Compatible brake fluid’’ the master cylinder diaphragm does not fit the cap! The fluid from Fuchs Silkolene is no longer available – did they know something? We advise against modern brake fluids because the seals in 1970s brake systems generally do not bear modern type (DOT 5 etc) brake fluids. Well any comments on this? Do I risk 5.1 instead of 4? I assume he means 5.1 as 5 is totally incompatible.