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97 GMC Sierra Hissing Brakes

I have a 1997 GMC Sierra pickup that hisses when I depress the brake pedal. It sounds like the sounds is coming from under the dash and the pedal sometimes is hard to depress. What might be causing this?

A slight hiss while the brake pedal is moving is normal. It’s how the vacuum booster works. However, if it hisses while the pedal is fully depressed, and/or the pedal is hard to press then you may have a failing vacuum booster.

How serious/expensive is replacing a failing vacuum booster? Am I able to drive the vehicle another day as I am unable to get it fixed today? Just curious but also concerned. thanks

It’s serious in that when you step on the brake, it may suddenly take way more force than you expected to stop the truck. (Way more as in both feet on the pedal and give it all you’ve got.) I’d at least verify that the parking/emergency brake works reasonably well, and then go slowly and follow at great distance.
Expense? I’m not sure, a wild guess is about $200 for the part and two hours labor. It could go higher than that. Sometimes the master cylinder leaks, and that causes the vacuum booster to fail, so you might need a master cylinder too.

My mechanic can get to it tomorrow and fix it for me. Thanks for the advice.