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re: Vacuum Pump experiment

>From: Brendan Rudack <rudack@ucsub.Colorado.EDU>
>Date: Sat, 16 Nov 1996 19:37:03 -0700 (MST)
>Subject: Vacuum Pump experiment
>Wanting a definitive answer about the brake performance with a
>"disabled" vacuum pump I took out the cam actuated rod.  After my
>brief test drive I determined that the vacuum pump for the vacuum
>assisted brakes has no functional pupose. 

Try running at about 115 mph (your car will do about 125 mph true, so
you are not quite WFO) gently step on the brakes, bringing it down to
100, then open it back up... , and then stomp (Like your radar
detector went off)  the brakes.  You will find your assit is
significantly reduced becuase of low vacuum at high loads.  (And
no, you don't get to know how I know this :) )  

You get into a similar situation if you are running a road course, 
lots of braking and hard acceleration.

> The brakes felt absolutely
>the same as before.  I don't understand why this is though.  In the
>manual it says that if the engine should die for any reason while the
>vehicle is moving there should be enough brake "assist" left to easily
>stop the vehicle once.  I tested this (rod in place)and sure enough if
>you kill the engine and hit the brakes they feel normal, but if you
>let your foot up then hit the brakes again the pedal is dead hard and
>it requires the strength of Thor to stop the car.  Now logically I
>would think that shutting off the motor stops the vacuum pump from

The pump *AND* engine provide vacuum to the booster.  There is a 
check valve on the line to the pump, preventing assit loss when the 
engine is shut off.  Look at the vacuum hose routing under the 
hood...The booster has enough volume to "store" assist for one stop.

>working therefore deleting the power assist to the brakes.  Now why in
>the name of the Audi gods do the brakes still feel fine with the rod

See above. (Check valve)

>Next question:  without the rod in place could there be any
>danger in "excess" amounts of oil collecting in the pump?

Can't think of one....

>Brendan Rudack			rudack@ucsub.Colorado.EDU
>'88 90Q
>Fenton, Michigan USA

I have had by pump rod removed for several years, with no noticeable 
problems.  But, I can't say that somebody, somewhere might find a 
problem with this condition.  

Allan D. Morris
Phoenix, AZ
1988 90Q