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I have this hissing sound that seems to be coming from just behind my brake pedal. When I apply the brake, the hissing stops. And when I get off the brake, it comes back. It doesn't seem to be affecting anything its just really annoying. Can someone please tell me whats going on?

is it coming from under the hood? may be the vacumm line on the break booster thats the only vacumm line in that area i think.

The brake booster is most likely going bad. It could be the vaccum line like FatMan said. Check that first. Make sure the line is not cracked around the plastic fitting where it attaches to the booster or that the rubber seal the plastic fitting goes through. In my expierence if the line is good, then the booster will need to be replaced.

About how much will a brake booster set me back?

It depends on where you get it. You could try taking the vacuum line off of the brake booster and plugging it to make sure if it is the booster leaking. You could also try using a vacuum pump on the booster and see if it will hold the vacuum. One more thing does the engine rev when you engage the brakes?

The engine doesnt rev when i hit the brakes. If I am at idle and I mash the brake hard enough, it seems as though it will die if I keep holding it.

I would try the other tests then. Especailly the bleed down test on the booster.

Last booster I bought for my 85 BII was $65 with core and no master cylinder. I think with the master cylinder it was $123. That was at Auto Zone. My 86 has a different size booster that the 85. Haven't had to replace it yet, so not sure if there is a difference in price between the two.

thats not too bad. I think thats what im gonna have to do. I dont have any vaccuum leaks so I think its the booster. That would also explain why I have no power brakes when the truck is turned off. Thanks guys.

Reinforcements wrote:That would also explain why I have no power brakes when the truck is turned off. Thanks guys.

I don't think anybody has power brakes when the engine is off.

I thought the booster held some vaccumm so if the engine dies or something, you can still stop easily

Not normally. One some vehicles that haven't been modified and are quite, that is how you can tell the motor is dead by the brakes are harder to push. Maybe if there was a vacuum reservor inline with the booster that might be the case, like in heater controls, but I doubt it.

Ah, alright.