Just bought a 72 40 and there are two concerns I'm hoping someone can direct me to a fix. The first is a hissing noise when stopped (engine on) and brake pedal down medium. When hissing, vacuum drops and engine runs rough. When I push hard on the brake, hissing stops, vacuum goes back up and engine runs smooth. Second is a whistling sound when vehicle is moving. It is coming from the engine/trannie and maintains a constant tone regardless of speed or gear. It stops when the vehicle is almost stopped (1-2 MPH).
Any ideas??

 

I had a similar experience for your hissing on my 71 '40. The rubber boot had come disconnected from the flange around the rob going through the firewall to the booster. I put that back in place, and no issues since. My engine didn't react to it, though.

Post this on mud and you will get more answers.