|Originally posted by LZeppelin513:|
Well i think i figured it out. I am pretty sure it is my brake booster. It holds a vacuum perfect after the car is off so i never suspected it. However, if the car is off and I hit the brake it leaks all the vacuum out as the brake depresses. That is why it feels like it has to build vacuum, because it does have to build a vacuum. When the car is running and i press the brakes it leaks out most the vacuum and then rapidly builds to assist. does this make sense or am i crazy.
That's actually how a booster is SUPPOSED to work. When you aren't wanting boost, the booster lets engine vacuum suck on both sides of the diaphragm. When you step on the pedal, there is a spring loaded valve in the booster that opens the "close to you" side to atmosphere, so that there is more pressure behind the diaphragm than in front of it, pushing it forward (against the master cylinder piston, providing assist) That behavior is normal.\
I do know what you mean about the "brake lag" sensation though. I have noticed that relative to a new car, it always feels like there is about a half second between sudden application of moderate pressure to the brake pedal and when the brakes feel like they finally start to "bite in".
I don't really know for sure what causes that lag sensation on older boosters, perhaps the sizing of the valve that lets atmosphere in is conservatively sized to prevent the brakes from being too "touchy"