97 Bravada brake issue
 

This probably isn't model specific, but I'll try it here.
For the past week or so my 97 Bravada has been having a problem.
When you apply the brakes it makes a "hissing" sound.
No brake light, no codes if it even reads brakes, no fluid leaks.
Just the sound, and sometimes you have to push harder to get it to stop.
I'm thinking there's a problem with the brake booster system.
Any ideas?
Thanks