I have a 95' S-10 Blazer, mileage 136,000 and when I press the brake pedal I hear a hissing noise and the brakes become really stiff, not to the floor like other forums I have read. I have been told that the brake booster needs to be replaced? Easy enough, but I was wondering if there was any chance it could be the breaker rather than the booster, and if so how would I check? I have no vacuum leaks under the hood that I can tell. Any help would be appreciated! Thanks!