Mercedes Benz repair: 1981 300D 123-130 vacuum problem, vacuum gauge, vacuum problem

Question
When i'm sitting at a stop light w/my foot on brakes my climate control stops working.the brake pedal feels hard after a couple of applications.i'm leaning toward brake booster but are there some other things I should check first?  i have vacuum at my pump, 26hg .Thanks for the help; Ben  

Answer
T into the vacuum supply line, attach a vacuum gauge with a long hose that will reach in the car.
Go for a drive and watch the supply vacuum, it should vary a small amount, unless there is a huge leak or a bad pump.

If the pedal does not fall down; but becomes rock hard, it is a bad booster.