The car is an 89 CRX Dx. Just today the car lost all brake assist. You have to press very hard to get the car to stop.
I think its the brake booster or the Check valve. I'm hoping you guys can help me out to determine which it is

When pressing the brakes while idling i can hear a very loud hiss of air coming from around the brake pedal area. It only hisses when pressing the pedal down while idling. If im not idling itll hiss just once and then be completly flat.

1. Is this symptom typical of a dead brake booster or is it the brake booster check valve? I'm leaning towards the booster since i can hear air hissing out in the pedal area.
2. How hard is it to replace the booster? Any how to's anywhere?