Hissing Sound When Brake Pedal pressed

[Canadian1968]

I have a 68 Charger RT 440 Bone stock except for a Holley 750.  When I press the brake pedal I get a loud hissing sound , that is very noticeable inside the car. The brakes work , but there is a bit of play in the pedal before the brakes grab. Fluids are up , I am getting almost 19" of vaccum from the engine.

Where should I start looking?

Thanks

[Dmichels]

Bad booster
Dave

[ACUDANUT]

That could be. Does thecar stop good. Is it hard to stop ?

[Dmichels]

I have had leaky ones that still stopped well But when you pressed the pedel it hissed at you
Dave

[Canadian1968]

Must be replaced or anything I can do to fix?

[Cooter]

Replace the master cylinder as well. Most likely leaking fluid into Brake booster, which swells diaphram. Must be replaced.