hissing from under the dash

Hi,

There is a good chance that is the problem. They can fail. Since they are actuated by engine vacuum, they can make a hissing sound. If the leak is bad enough, it could cause an idle or drivability issue.

If you look at the pics below, they show the directions and pics related to replacing the vacuum brake booster. When you complete the process, make sure the vacuum hoses are properly installed and not leaking.

Check out the diagrams (below). Please let us know if you need anything else to get the problem fixed. Joe