Brakes not very good..... :(

I was backing the Dart out of the shop today so I could do a little work to another one of my project cars. I personally have not driven the Dart any at all except backing it out of the shop today, but I noticed that when I press the brake pedal the engine will start missing. I know that is usually some sort of vacuum leak. The brakes were fine a few weeks ago when my Dad and I had the car out for state inspection?? Anyway is there a way to test the brake booster??? Any other suggestions on tracing the issue??

The car had 4 inch (sbp) drum brakes. We swapped over to lbp 4.5 inch disc on the front.