Of vacuum brake boosters

["Ernie Stepney" <estepney@xxxxxxxxx>]

Brad:
I think you are confusing the 58 booster and a conventional vacuum
suspended booster. The bellows is at atmospheric pressure until you open
the vacuum valve. It works against the brake pedal linkage rather than
running the pedal force through the booster piston.  

Ernie
(58 LeBaron on the road Saturday)

-----Original Message-----
From: mailing-list-owner@xxxxxxxxxxxxxxxx
[mailto:mailing-list-owner@xxxxxxxxxxxxxxxx] On Behalf Of Brad Hogg
Sent: December 18, 2003 1:02 PM
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: Re: IML: Fw: Of vacuum brake boosters


You are exactly correct Hugh.

There are two chambers involved here and four "seals".  You have dealt
only with the bellows inside the chamber.

There is a seal at the front, where the rod passes through the front
housing of the booster.  There is a seal in the centre (mor accurately a
valve) where the rod passes through the bellows.  There is a seal at the
rear where the rod passes through the rear of the booster. There is the
bellows themselves causing a seal near the centre of the booster.

If the bellows themselves leak, you are correct in that nothing will be
noticed until you depress the brake pedal.  At that point, rather than
vacuum being maintained in front of the bellows and atmospheric pressure
allowed to act onthe rear of the bellows to create "assist", the effect
will be somewhat deminished and if the leak is sufficient, the engine
may stumble due to a lean mixture condition on cyl 8.(vacuum tree is
located nearest cyl 8 on 440, YMMV)

If the seal in the centre of the unit leaks, you'll see much the same
effect unless the issue is compounded by a rear sea leakage, in which
case you will feel very little assist and will notice engine stumble
continuously.

If the front seal leaks, you will feel diminished assist and will notice
engine stumble continuously.  I recently "patched" up this problem on
one of my cars in a way that seems to be working perfectly for the past
5 months or so.  I separated the master cylinder from the booster and
pulled it ahead about an inch.  I then laid a nice thick gasket of black
RTV sealant onto the face of the booster.  I pushed the master cylinder
back into place and bolted it down.  This ended my engine stumble and
improved brake assist. This is of course a patch to a problem that was
never solved.  I do have intentions to replace the booster at some
point.

----- Original Message ----- 
From: "Hugh & Therese" <hugtrees@xxxxxxxx>
To: "Imperial Mailing List" <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Thursday, December 18, 2003 12:24 PM
Subject: IML: Fw: Of vacuum brake boosters


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