For starters, I know right off the bat that my HVAC controls hissing is a vacuum leak, which I intend on repairing (I found a great write-up on here detailing how to fix the issue). But I'm also noticing that my fuel gauge is at a quarter, but I know for a fact that I have a full tank. I also noticed that when I hit the gas at highway speeds, the faster I go, the HVAC hissing gets quieter. I'm also noticing that it feels like the car has much less power. I hear a powerful engine and high RPMs but the car feels like it's just not all there. Are all of these symptoms from the hissing HVAC controls?