Brake vacuum problem

I have a 96 s10 mi 120,000 when I first start the truck everything is fine after a few minuites of normal driving the brakes start to fail. Sometimes the petal is stiff with little of on brake application and sometimes the brakes grab. Also when ideling when the brake petal is pressed the engine idles up when the petal is released oil pressure drops and the engine dies. When the petal is held down and the engine is running a blowing noise is herd fron behind the brake petal. I would appriceate a little input on this problem thank you.