khankohati

Hi Guys,
For the last couple of months I hear hissing sound near the brake pedal when I press it. This does not happen in the morning when I start driving it. Only after driving couple of times that I hear that noise from the brake pedal when I apply brakes. Mechanic says that brake booster is bad. Is he right. I had my brake master cylinder changed last year by another mechanic.I changed front brake pads myself last May. Can this noise be coming due to change of master cylinder or brake pads or mechnic is right that booster is bad. He said don't worry about it right now.Once brake pedal start becoming really hard then I should change booster.
Any advice from anyone

Thanks