[Bob B.]

Hissing booster is not normal, may have a blown diaphragm. This can cause the vacuum gauge to drop to zero when applying the brakes as well. The tank behind the running board is a vacuum reservoir. As for the sinking brake pedal, could be air in the brake lines. These systems usually need to be bleed with a pressure bleeder because it is hard to get all the air out of dual rear wheel cylinders. All four wheels and drums should be pulled and the brakes completely inspected and adjusted, sometimes excessively worn drums and shoes can feel like air in the brakes lines. Since this is a medium duty truck and it has a single circuit master cylinder, if you are not completely familiar with how the brakes work on it you should have a competent medium/heavy truck mechanic check it out. Good luck, it looks like a nice old truck.