Hey all-I've rebuilt the drum brakes and put a new master cylinder on my Jetstar, but recently, when I start it up, I get a fairly loud hiss coming from near the brake booster. Sounds like a vacuum leak, but I cannot pinpoint it. When i depress the brakes, the hiss goes away. Is this a leaking brake booster probably?

Sounds like the inlet valve is leaking. There's a concentric inlet valve around the pushrod between the booster and brake pedal (labeled "poppet" in this drawing). With the pedal released, this valve should seal so that the volume behind the diaphragm can be evacuated by manifold vacuum. When you press on the pedal, this valve opens and the vacuum valve in the center of the diaphragm closes, so atmospheric pressure can enter behind the diaphragm. Since you still have vacuum on the front of the diaphragm, this is where the "boost" in the brake booster comes from.