Join Date: Nov 1999
Location: Central California, USA
On manual brakes there is a rubber boot on the firewall. This boot goes tightly around the pushrod that connects to the brake pedal.
On power brakes there is no boot on the firewall.
On power brakes there is a plastic bellows tube that connects the rear vent hole in the booster to the passenger compartment.
When you step on the brake pedal you are opening a valve that allows atmospheric air to come from inside the car and into the rear portion of the booster, pressing on the diaphram. Usually you hear a whoosh, when steping on the brakes, that's the air going into the booster right around the pushrod.
If you block the air from entering the rear of the booster you will not get any boost.
Several people who have posted here have added power boosters and left the firewall boot covering the booster rear opening and had no boost until they removed the boot.
The older I get, the faster I was!
[This message has been edited by davidpozzi (edited 06-17-2000).]