brakes, air in line and stalling when brakes are applied

What is your question?

when i went to start my blazer and i put my foot on the brakes it made a hissing noise and it caused me to have to brake alot sooner. then a couple weeks later it started to have a new problem with it with the hissing noise and now when i apply the brake the vehicle will almost or will stall. i need to know what is wrong with it. i tried to bleed the brake lines and the abs but still hissing and stalling. is it a brake problem, egr or something else?