Pontiac Repair: 92 Pontiac Bonneville hiss when heater on high and braking, pontiac bonneville, power brake booster

Question
Today I was driving my 92 Bonneville SE.  I had the heater on high and was coming up to a stop, and when I applied the brakes I heard a loud hissing when the brakes were about halfway through their normal range of travel.  This did not occur with the heater off, or with it on a lower setting.  What might be the problem?

Answer
Hi Brandon.

This is a wired issue here. The hissing sound you hear has to be some sort of a vacuum leak. I'm not really sure what corolation the brakes and heater on his have to do with anything. It's either one of the vaccum mode doors in the dash leaking, a vacuum line leaking the control head leaking or maybe even the power brake booster. Maybe with the right engine speed and load on the engine the vacuum level is just right in the engine to make the noise happen. It's going to take some detective work and another person stepping on the brake pedal while you listen under the dash for the source of the noise/vacuum leak. Good luck :)