The brakes hiss when I hit the brake pedal, and there’s no pressure at all. It hardly stops until I put all my weight on it. The rpm’s raise a bit when I hit the brakes, I was thinking maybe a vacuum leak?

 

Check the large vacuum hose closely that goes to the booster for leaks and to ensure it's not soft and sucking in on itself.

 

Your brake booster isn't boosting. Like another poster said, check the vacuum hose for collapsing or cracks. If there's a problem with the vacuum source, the booster doesn't have anything to work with. If everything is fine, the booster might be bad. they can be a pain to replace, plus expensive, so make sure it really is the booster.

 

Booster is leaking.