The high mount reservoir looks like the simplest option. You could also have a faulty or mis-labeled residual valve. Think about how the hydraulics work- - - -if the caliper piston has an area of 3 square inches, for instance, 2 pounds of line pressure will put 6 pounds of push against the brake pad. If the residual pressure goes up to 10 PSI, that push on the pad becomes 30 pounds of force. How about temporarily installing a 0-50 PSI pressure gauge in the front brake line and actually measuring the residual pressure? Just don't press the brake pedal hard, or run the engine to make the booster work- - - -you could damage a gauge that reads low enough to get a usable pressure reference.
Jerry
