Brakes make a hissing sound

[supercharged98]

Hey guys,
Just test drove a 98 Daytona Edition and it was awesome except for the brakes. When parked and I press the brake pedal I can clearly hear a hissing sound under the dash like a air leak . When driving the pedal goes down a little bit and gets very hard to press and the car doesn stop like it should. The ABS light is on.
I wanna buy it but I'm not sure what the problem is.
The car has 125k, body and interior are mint. engine sounds good and doesnt burn or leak oil. Trans fluid is a little dark but shifted fine.
They are asking $2800. I think its a great deal but I'm not sure about the brake problem.
Any help would be greatly appreciated
Thanks

 

[Fivefingerdeathpunch]

that is a good deal, as for the brakes I dont know what it could be

 

[imdaman3]

sounds like the brake booster is bad. buy it and fix its easy to do.

 

[matt5112]

Search impala brake booster for the write up on how to change it.

Have the same issue but i'm too lazy to change it. (no ABS light, and yes i've pulled the fuse to see if the light still works, it does).