98 (base)= brakes?? hishing noises?

1998 base model,3.1
about 232,000 miles on it.

purchased at about 45,000 miles and always had a slight "hishing " noise when I apply the brake.

But=--

yesterday the car was parked on a very slight incline.
emergency barke was appllied. a/c was on .
I returned about 5 minutes later and heard a "hishing" type of sound.
it seemed to be coming from under the dash on drivers side.

As I put my foot on the brake and shifted into reverse the noise stopped.
Today at various times I have the same noise.

I,ve owned at least 40 cars in the last 50 years and this car has the very worse brakes of all . I,ve tried all types of pads ,etc. The rear cylinders rebuilt about 150,000 miles ago.Master cylinder is original.

As always
Thanks for any advice.

First Guess: Defective brake booster.

I'm told that the boosters used after '94 or '95 are an improved version compared to the '93 and '92. Both my '93 and '92 had to have the boosters replaced due to poor braking power--although neither one was noisy.

I bet your noise goes away, and your braking power improves with a replacement booster.