When faced with a leaking brake valve, many adventurous home mechanics follow the same path I did and wonder if it's possible to rebuild the valve. Many valves can indeed be rebuilt, but it's important to understand that it can't usually be done using seals found at your local parts store. That's because the Nitrile/Buna-N or Viton (a fluoroelastomer) rubber compounds used in o-rings and other seals found in "over the counter" kits aren't compatible with automotive brake fluid - they're designed to be compatible with petroleum-based fluids like engine oil and gasoline! When exposed to brake fluid they tend to soften and swell, which leads to compromised seals, leaks, and component failures. Do you really want to deal with these issues when working on your brakes?
The most commonly used compound used for compatibility with automobile brake fluid is ethylene propylene diene monomer, or EPDM. Muscle Car Research uses only EPDM rubber in all of our brake valve repair kits. You can buy from us with confidence knowing that our products are fully compatible with both glycol-based and silicone-based automobile brake fluids.
Here's how the DuPont General Chemical Resistance Guide describes the compatibility of various rubber compounds with automobile brake fluid (click on the image for a larger view).