1998 Toyota Camry hissing sound when pedal is depressed

I recently noticed a hissing sound when the brake pedal is depressed, it lessens or stops hissing when I have the pedal fully depressed hard (like at a stop light). The brakes seem to work fine. But a couple of times when I had to stop fast it seemed like a lot more pressure was needed. As if the brakes weren't working as well as they should. I had brakes and rotors replaced at the dealer a few months ago. Am I in for another huge repair bill?