Well first of all it may not be the Booster,it might be the vaccum pump.First thing i would do is remove the line from the booster and look inside the booster and see if you can see any fluids(brake fluid or oil)if none good.next start the car and feel if you have any vaccum on the line you just removed.(oh when you removed the live you should hear a hissing sound,witch is the vaccum escaping ,that is what you want) If you have a nice suction going on the vaccum pump might be good,but if you have low suction than the valves inside the pump and you need a new one or rebuild it,parts are availabe for that.If you have good suction than it is a possability that it is the booster.To check the booster hook the line back up and let the car run for a while .shut the car off and pump the brake pedal about 10 times after the last time keep the foot on the pedal trying to brake and start the car,now the pedal should go down further to the floor ,and if it does the booster is good if it dosent it is bad
Also make sure that the vaccum line that you removed is not brocken .Maybe you are asking yourself you dont have a vaccum pump.Yes you do,a diesel motor does not create enough vaccum so they install a pump . Well good luck Harald