1990 D150

[figman]

When I apply my brakes I hear a sound as if I have a vacuum leak. Could this be my brake booster or is this normal for this truck? Has anyone had this problem? If so, any suggestions?

[six pack]

Sounds to me like you have already diagnosed it.
If you hear a constant hissing while holding the brake pedal steady at any point while applying the brakes, you have a bad booster.

Hope this helps:)