My brake pedal is making a hissing noise. I checked my brake fluid, thinking I might be low; but it was fine. Is my master cylinder going bad??
It sounds like you have a hole in the diaphragm of your brake booster ( the large round canister your master cylinder is bolted to. )
How much am I looking at in repairs?
I agree that the brake booster is probably leaking.
Depending upon the region where you live, replacement will probably cost anywhere from about $250 to $300 for parts & labor.
Agreed. That’s with a remanufactured booster, which is perfectly fine to use.