96 Jimmy SLT brakes making noise

I have a 96 Jimmy Slt. When driving home today it started making a "swooshing" or "sucking" noise when pressing the brake pedal. Also when I was sitting at a stop light for an extended time after holding the brake down, the truck sounded like it was going to stall as i accellerated.

Any help diagnosing this problem is greatly appreciated.

Thanks in advance.

Vacuum booster leaking?

It was the vacuum, power booster. A repair service wanted $600 to fix it including parts. I went to Auto Zone and bought a reman power booster for $70 and did it myself. It took over 3 hours to do, using hand tools. Thanks for your help. Hopefully I wont be needing anymore help anytime soon.