2001 Sequoia Limited 4X4 - the brake booster is hissing loudly and diminishes when the brake pedal is pushed or engine speed increases. Does anyone know just what is defective and what part is required to correct it? Is it a simple seal or the complete booster? I cannot find breakdown diagrams of the booster components, so it remains a "mystery". The braking performance appears normal, so I wonder just how serious this leaking is to overall function. The vehicle has skid control, which is important when considering the booster replacement.
Any thoughts would be appreciated.