This may seem like a goofy question to some, but really, I have no clue as to how a power assist brake booster operates. What makes it do what it does? Anyone have a simple mans explanation?
Thanks,
Patrick

I'm not 100% sure, but there is a rubber diaphram inside which has a lightweight spring pushing it away from the master cylinder. So, when the engine is running the diaphram is closest to the firewall being held by the spring pushing it back (from the front of the car) and the area between the diaphram and front of the booster is under vaccuum from the engine, but the vaccuum isn't strong enough to pull the diaphram into the master cylinder. So when you press the brake pedal, since the diaphram is under vaccuum pressure to be pulled into the master cylinder it helps push the piston of the master cylinder in when you hit the brake pedal.

That's how I 'think' it works, I could be wrong.

Actually I believe that it starts out with vacuum applied to both sides of the diaphragm. Thus the light spring can push the diaphragm back towards the firewall.

When you push on the pedal you actually change valve positions and open the back side of the diaphragm to atmosphere, thus the vacuum pulls the diaphragm in adding mechanical assist. I would guess that the valve only stays open until the diaphragm "catches up" with the brake pedal, probably with some amount of bias in the valve control springs.

You could do a google search and find out more... or cannabilize one to take apart :-)

here you go...
http://auto.howstuffworks.com/power-brake1.htm
david