How much vacuum at idle measured at the brake booster?

Brake pedal went hard but was able to stop, eventually. Shop diagnosed a bad booster and replaced it. Still had same symptoms. They declared the new booster defective and got another one. Still had same symptoms. At this point I got in the diagnostic loop and we found that heater controls also did not work properly. They put a gauge in the vacuum line and declared that there was a vacuum so the pump was ok. It seemed to me that the vacuum was too low and did not build up very fast when engine started. So the question is, how much vacuum should the gauge sho at idle?

Thanks,
Steve

 

Im sure on how much vacuum you are supposed to have at idle but you have the sypmtoms I had when my pump went.