vacuum with blower

[Gary Derian]

(lots of snipped messages regarding check valves for brake boosters)

Even though the engine can run under boost at full throttle, there is still
a throttle which is partially closed under cruise conditions and the engine
operates under vacuum during this time which is most of the time.  During
3/4 throttle or so, there can be vacuum between the throttle and turbo yet
boost between the turbo and engine.  The vacuum is stored in the brake
booster and also in auxiliary reservoirs to operate the air conditioning
ducts in the dashboard.  Even without a turbo, all cars should have a check
valve in the brake booster.  Cadillacs from the early 80's had an electric
vacuum pump to assist the brake booster and other minor vacuum controls.  

Low vacuum and poor brake boosting has caused many manufacturers to seek
hydraulic brake boosters.  GM used a Hydroboost on Diesels and station
wagons (still used on Hummvees), a power steering pump driven hydraulic
brake booster.  This turned out to be a bad idea since power steering
hydraulics have an open center valve which constantly circulates fluid. 
This meant the hydroboost had to be plumbed in series with the steering
gear.  Every time the brakes were applied, the steering tugged a little. 
Later GM cars used a Powermaster, an electric driven hydraulic brake
booster, used on Grand Nationals and station wagons.  This was a good idea
but was bad execution.  ATE had a similar system on several German cars and
Audis used a power steering based system with its own circuit.

Gary Derian <gderian at cybergate.net>