Rob and Emery,

Thanks for the detailed information. I assume the part from Dave Lenzi to reduce or eliminate vacuum pump cycling is this from his web site:

"Air Manifolds with Schrader valves, ball valves, S.S. gauge and fittings. "

Is this right or would I also need to replace my vacuum pump with Dave's?

As an aside, I replaced a perfectly good vacuum reservoir with the vacuum pump. I got 6-7 applications of the brakes with the reservoir. It is now unused on a shelf side in my garage. What do you guys think about a vacuum reservoir?

Off subject grumbling but kind of related: I sometimes wonder if some "upgrades" are, in fact, progress. The situation is slightly similar to the xenon headlights on our last car. I couldn't tell that they were any better than the $10.99 Halogen headlights on our other car. It cost $1,600 to fix the xenon system when one caused both to go out. Fortunately, at the time, the car was still under warranty. It didn't happen again but one light was dim when I later sold the car.

Tom Whitton
26 foot upgraded GMC
Paducah, KY

Message: 9
Date: Sat, 31 Jan 2015 16:38:01 +1100
From: "Robert Mueller"
To:
Subject: Re: [GMCnet] vacuum booster MC seal (Tom Whitton)
Message-ID:
Content-Type: text/plain; charset="us-ascii"

Dave / Emery,

If you take the heads off the Ford vacuum pump (as I have) you will find flapper (check) valves that control the flow through the
pump, they are not a perfect seal and will leak. They are under the lumps in the heads:

http://www.gmcmhphotos.com/photos/vacuum-pump-for-brakes-lenzi/p55346-pump-mount.html

The reason Emery's pump does not cycle is because his switch is mounted in the manifold Dave makes. It plugs directly into the
booster and the check valve plugs into that manifold.

http://www.gmcmhphotos.com/photos/vacuum-pump-for-brakes-lenzi/p55345-pump-mount.html

A hose connects the outlet of the booster check valve to the inlet on the pump. The switch senses pressure in the booster and only
comes on when the pressure in the booster goes above the set point of the switch. The pressure in the line that connects the pump to
the switch can go to atmospheric pressure and as long as the pressure in the booster is lower than the switch set point the pump
will not run.

However, if one does not have the manifold that Dave makes and the switch is installed between the booster check valve and the Ford
vacuum pump the flapper valves in the pump will leak and allow the pressure in the line to increase and the pump will come on. It
will run until the pressure in the line is below the switch set point and it will switch off. This cycling will occur whether or not
you actuate the brakes.

The reason Dave made the manifold is because people complained that their pump was cycling.

Regards,
Rob M.
The Pedantic Mechanic
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426

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The source of my vacuum leak was at the front of the booster. The master cylinder must center in the booster. Dave made a nylon (?) part that slides over the end of the master cylinder. This keeps the push rod centered. The plastic bushing then presses against a rubber seal that slides into the opening in the booster before the master cylinder is mounted.
Many systems are missing that rubber seal. Mine was missing and I bought mine from a Corvette online parts place.

Emery Stora

> On Jan 31, 2015, at 11:01 AM, Tom Whitton wrote:
>
> Rob and Emery,
>
> Thanks for the detailed information. I assume the part from Dave Lenzi to reduce or eliminate vacuum pump cycling is this from his web site:
>
> "Air Manifolds with Schrader valves, ball valves, S.S. gauge and fittings. "
>
> Is this right or would I also need to replace my vacuum pump with Dave's?
>
> As an aside, I replaced a perfectly good vacuum reservoir with the vacuum pump. I got 6-7 applications of the brakes with the reservoir. It is now unused on a shelf side in my garage. What do you guys think about a vacuum reservoir?
>
> Off subject grumbling but kind of related: I sometimes wonder if some "upgrades" are, in fact, progress. The situation is slightly similar to the xenon headlights on our last car. I couldn't tell that they were any better than the $10.99 Halogen headlights on our other car. It cost $1,600 to fix the xenon system when one caused both to go out. Fortunately, at the time, the car was still under warranty. It didn't happen again but one light was dim when I later sold the car.
>
> Tom Whitton
> 26 foot upgraded GMC
> Paducah, KY
>
> Message: 9
> Date: Sat, 31 Jan 2015 16:38:01 +1100
> From: "Robert Mueller"
> To:
> Subject: Re: [GMCnet] vacuum booster MC seal (Tom Whitton)
> Message-ID:
> Content-Type: text/plain; charset="us-ascii"
>
> Dave / Emery,
>
> If you take the heads off the Ford vacuum pump (as I have) you will find flapper (check) valves that control the flow through the
> pump, they are not a perfect seal and will leak. They are under the lumps in the heads:
>
> http://www.gmcmhphotos.com/photos/vacuum-pump-for-brakes-lenzi/p55346-pump-mount.html
>
> The reason Emery's pump does not cycle is because his switch is mounted in the manifold Dave makes. It plugs directly into the
> booster and the check valve plugs into that manifold.
>
> http://www.gmcmhphotos.com/photos/vacuum-pump-for-brakes-lenzi/p55345-pump-mount.html
>
> A hose connects the outlet of the booster check valve to the inlet on the pump. The switch senses pressure in the booster and only
> comes on when the pressure in the booster goes above the set point of the switch. The pressure in the line that connects the pump to
> the switch can go to atmospheric pressure and as long as the pressure in the booster is lower than the switch set point the pump
> will not run.
>
> However, if one does not have the manifold that Dave makes and the switch is installed between the booster check valve and the Ford
> vacuum pump the flapper valves in the pump will leak and allow the pressure in the line to increase and the pump will come on. It
> will run until the pressure in the line is below the switch set point and it will switch off. This cycling will occur whether or not
> you actuate the brakes.
>
> The reason Dave made the manifold is because people complained that their pump was cycling.
>
> Regards,
> Rob M.
> The Pedantic Mechanic
> Sydney, Australia
> AUS '75 Avion - The Blue Streak TZE365V100428
> USA '75 Avion - Double Trouble TZE365V100426
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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