new front brakes on 97 expedition

[scrambledlife]

I did change the brake pads in the front of 97 expedition but that hissing noise is still there,my guess I have air in the line.Do I? Or should I to replace the power brake booster.
Oh,by the way when the car is off brake pedal is hard.ty

 

[tonydiv]

If you have a hissing sound, it is probably coming from the booster or the vacuum line to the booster. There is also a check valve that goes between the booster and the vac line that could be at fault.

Since the pedal is hard when the truck is off, I don't think that the master cylinder is bad or that there is air in the lines. So I would focus on that booster.

 

[scrambledlife]

Looking around seems like is the booster,so should I go ahead and change it.And you are right is not MS.I looked at the booster from inside on fire wall and there no bolts on them,that could not be the problem with booster?ty