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1997 F-150 hissing?

My truck makes a ssssss’ing noise when I drive it. The noise increases in speed with the speed of the vehicle. Also, when I apply the brakes the noise stops. Then when I accelerate, the noise starts again.

The hissing might be from a defective vacuum brake booster check valve, or from a leaking diaphram within the vacuum brake booster.

To determine if it’s the check valve, remove it from the vacuum hose and the brake booster. Now try blowing thru and sucking back on the check valve. There should be a restriction in flow one way or another. If there’s not, the check valve is leaking.

If the check valve indicates that it’s not the problem, then most likey the problem is with a leaking diaphram within the brake booster which means it requires replacement.


I agree with Tester.

Me too.

The booster diaphragm can be checked easily by pumping the pedal three or four times with the engine off (you should feel the pedal get hard) and then, while holding the pedal down, turning the engine on. The pedal should sink a bit and the feel should soften. If there’s no change, the booster is bad.

Whoops, this was supposed to be attached to William’s reply. Eventually I suppose I’ll get used to this newfangled technology…