The aluminum (not plastic) cylinder inboard of the HB unit is the reserve accumulator. DO NOT mess with it. It retains full system pressure (until it bleeds down, or is depleted by brake pedal operation with engine off). They all weep a little fluid at the base. I've never seen one that didn't, within a year or two of new. If you find one that isn't moist with fluid, you've found a rare jewel, or the system is dry (not unlike Detroits and oil leaks).