Originally Posted By tman
The booster is definitely suspect, but also check the stems of the plastic check valve. My check valve was a single stem. That stem was cracked at the base thus a loud hissing. Changed out the check valve and all was good.
Good point, check valves can be purchased through the Ram Man. He has a good video on boosters as well.

http://www.theramman.com/component/virtuemart/b-body-components.html