Help before this drives me insane.

[NE Ohio 911]

I replaced my master cylinder last winter. Everything has been fine until a month ago when I started hearing an intermittent hissing sound coming from behind the brake booster. After reading some posts on PP, I replaced the vaccum hose, check valve and grommet but it didn't fix the problem. If I plug the hose the hissing stops. I guess the only thing left is the seal between the booster and master cylinder? Brakes are fine and the hissing does not change when engaging them. Any input would be apprecited.

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[yelcab1]

That booster needs rebuilding, or replacement. Go to a reputable rebuilder or you will not be happy. Or, just buy a new one.

[Locker537]

Diagnose this properly before replacing something expensive like the booster.

Two things you can try to isolate where the "hissing", which we assume is leaking air, is coming from.

1. A cheap mechanics stethoscope - move it around until the sound is loudest to find the leak
2. Soapy water in a spray bottle - spray the area and look for bubbles

If you are referring to plugging the short hose near the booster, by removing the end that connects to the inlet on the chassis, and the hissing stops, my gut reaction would be something is wrong with the check valve. Your finger is acting as the valve instead of the actual check valve. Ensure it is installed correctly. It's hard to mix up but not impossible.

[brshap]

Hissing is a classic sign of a booster on it's way out.

[gomezoneill]

Use a vacuum line and put it up to your ear. Cheap man's stethoscope.

[NE Ohio 911]

Thanks for the suggestions everyone.

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