It sounds like a stuck valve in the power booster. Look up under the dash where the brake pedal push rod goes through the firewall. If that's where the hissing is coming from, the valve is stuck / broken or the diaphragm is ruptured.
If the hissing is heard better from under the hood, look for a 3/4" diameter hose that's plugged into a black or light yellow plastic check valve right behind the master cylinder. If something became disconnected, not only will you lose power assist, the engine should run very poorly with such a huge vacuum leak.
A competent do-it-yourselfer can replace a booster. Don't worry about getting one from a salvage yard because they are a very low failure item. What I would watch though is to always hold it in the installed position when you transport it. Some GM units have strange valves that fall out of position if the booster is turned sideways. They have a very high failure rate. I've never heard of that problem with Chrysler boosters, but why take a chance?
Caradiodoc
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Friday, May 29th, 2009 AT 4:17 AM