in response to a message on the NCRS # 18957
.? 66 fairly loud, very annoying noise coming out of the backside of my PB booster when I depress the brake pedal

Tuesday, 29 May 2001

The PB silencer is missing, defective or displaced
On 63-67 Corvette vacuum power brakes, the air is sucked out of the front of the booster via the intake manifold. This air supply enters the rear of the booster, located under the dash. This in rushing air is filtered via a foam filter in the rear of the booster. This air must first pass through an additional item called a silencer. The silencer is a 1/4” thick disc of dense felt about 2” in diameter. It is held in place by a hair pin type retainer. When you step on the brakes, air rushes into the back of the PB to replace the air sucked out the front. This partial vacuum produces the vacuum assist to the brakes.

Air rushing across an open tube can produce sounds, such as blowing across an open bottle of pop. Air rushing into the rear tubular piston of the PB can also produce sound. I suspect either the silencer (i.e. felt) has deteriorated and is no longer effective or it fell apart with age. Or the hair pin retainer has slipped out and allow the silencer to move down the shaft where it no longer covers the rear opening of the PB booster properly.