I have to start a new thread, as the previous message is old. No, it is not the cruise control switch that emits the hissing noise. It's the sound of air sucked back into the booster from under the dash when the pedal is applied. I have no idea why my brake booster is failing. The check valve is fine, the vacuum hose looks OK, and the boot around the piston looks clean. Other than the hissing noise, the idle goes down a little (a new symptom today). I would appreciate any advise on how to replace the booster.

Questions:
1. In order to remove the m/c, the two brake lines under the m/c need to be removed. Should I empty the brake fluid reservoir before unbolting the lines?
2. Will I introduce air into the m/c or disturb the hydraulic system?
3. If I did, could I push the air back into the reservoir by pushing the caliper piston?
4. Is there any special tools needed to remove the m/c from the booster?
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'96 960 89K