Of vacuum brake boosters

["Hugh & Therese" <hugtrees@xxxxxxxx>]

There is a gap in my understanding that makes me puzzled every time a
certain answer is given.  A member has a 1959 with a failed brake booster.
The type resembles a rubber concertina, connected directly to the brake
pedal.  The way it should work is as follows.  Once the engine is turned on,
vacuum is created in a largish tank on the interior side wall of the car,
adjacent to, but separate from, the actual brake booster,  When the brakes
are applied, a valve opens in the booster itself and the air within the
rubber concertina is rapidly pulled out by the tank, which is held at vacuum
by the engine just for this purpose.  When the brakes are released, the
valve is closed and vacuum is discontinued within the bellows unit, allowing
it to return to its former position, full of air.

 I hope this is an effective, if somewhat crude, way of explaining its
operation.

 So, I am puzzled when it is stated that a leak or breach in the bellows
unit
is given as a reason for an engine to run rough at idle.  This runs counter
to my experience.  When I has a similar breach in mine, it had no effect on
the engine unless I pressed the brake pedal.  I surmise, apparently
incorrectly, that with the valve in the booster unit closed, vacuum in the
separate tank is just fine, and the breach in the bellows unit is not
affecting the engine because it is not in use, or not part of the system,
until the pedal is pressed in and the valve is opened.  At that point,
obviously, the leak is pretty bad and all vacuum is lost.  Prior to this
action, however, surely there is no vacuum in the bellows unit anyway, so a
breach in the unit cannot affect the engine.

I am always thrown by folks saying their booster unit leak is affecting
engine performance.  I cannot see how this can be and look forward to
learning how what seems to me to be improbable is actually taking place.
Also, I wonder why the breach that I have had in my bellows unit never
affected the engine until I pressed the brake pedal.

 Hugh
> 58 Imperial, with a newly replaced bellows unit
>