pinch off the rubber vacuume line between the servo and the engine. The servo leaking is a major leak and cutting this off should make the engine run a lot smoother, or at least change the idle speed.
Also take a careful lap with the booster vacuume line pinched off. and see if the brakes work harder (You have to apply more perssure to stop) than with the line open. If there is no noticable change then the booster is probably bad.(it wasen't doing anything when it was hooked up)
I've never had an MGB booster apart, but I know that most other boosters contain enough spring tention to seriously injure you if you don't open them correctly. That said, I would not reccomend trying to open it up to rebuild it.