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Re: [GMCnet] Ford A426C brake vacuum pump, your opinion please? [message #327580 is a reply to message #327572] Fri, 29 December 2017 16:14 Go to previous messageGo to previous message
Ken Henderson is currently offline  Ken Henderson   United States
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
Senior Member
IMHO, the most reliable and economical backup for vacuum brakes is a large
reservoir, such as a 30" section of 4" pvc pipe mounted inside the GMC's
front cross member with wire ties. Properly constructed and installed,
such a device should hold enough vacuum for 2-3 fully assisted brake
applications at near the ambient atmospheric pressure. That assumption is
based on the likelihood that since the last previous brake application
(including the current stop) the engine will have been providing its full
vacuum capability several times. With the engine braking at closed
throttle the vacuum it generates should be nearly as good as a
purpose-designed vacuum pump. Installing a vacuum gauge and monitoring it
during normal driving will show that full vacuum occurs at almost every
significant deceleration event -- the reservoir will capture and retain
that vacuum.

Achieving that "proper construction and installation" may not be as easy as
it sounds. I've made quite a few of them and found that leak-proofing can
be difficult. There must be a check valve at the "entry" to the reservoir
(the connection to the engine). I found that many new off-the-shelf
booster check valves leak; my best source proved to be the junk yard. More
than one check valve removed from long-stored junkers released vacuum when
pulled from their boosters. The hose connections to the reservoir and to
the various components, including the check valve at the booster, and its
mounting grommet, MUST be snug & air tight. Vacuum is easily destroyed by
the smallest air leak.

This is not to denigrate the value of an electric boost pump, but absent
that more expensive improvement, the reservoir is a vast improvement over
just booster reserve. I always ran both.

​Ken H.​

On Fri, Dec 29, 2017 at 3:50 PM, James Hupy wrote:

> A running GMC engine in good shape, at sea level, will make in excess of 25
> inches of vacuum at 30 mph with your foot off the gas. Like when you might
> be using the brakes. It will make 2 or 3 times more vacuum than your coach
> brakes will ever need.
> So, now we have established that a running coach really does not have
> any need for an auxiliary vacuum pump, we can deal with "Yes, but, OMG,
> what if the engine quits?" The booster has enough reserve vacuum for you
> to bring the coach to a safe stop, ONCE. If the GMC needed an auxiliary
> vacuum pump, why didn't GMC install one from the factory? Because it was
> not needed? Don't know for sure if that is the real reason, but, it seems
> likely to me. A hydro boost system is also reliant upon the engine running
> to be able to multiply the pedal effort. No back up system there, either.
> Seems to me that it would make good sense to insure that the fuel
> tanks are not close to empty, and that the engine is in good enough shape
> that it does not stop running except when the driver wants it to. But, it's
> your money, spend it how you want.
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403
>
> On Dec 29, 2017 12:15 PM, "Rob Mueller" wrote:
>
> Mike,
>
> Below you will find a couple of emails I sent in earlier this month.
>
> Regards,
> Rob M.
> The Pedantic Mechanic
> Sydney, Australia
> AUS '75 Avion - The Blue Streak TZE365V100428
> USA '75 Avion - Double Trouble TZE365V100426 USA '77 Kingsley - TZE
> 267V100808
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Rob
> Mueller
> Sent: Saturday, December 9, 2017 11:43 PM
> To: gmclist@list.gmcnet.org
> Subject: Re: [GMCnet] Ford Azure / F-150 Eco Boost Vacuum Pumps
>
> G'day,
>
> Additional info on the F-150 Eco Boost Pump:
>
> RockAuto P/N: MOTORCRAFT BRPV30
>
> Link: https://www.rockauto.com/en/parts/motorcraft,BRPV30,vacuum+pump,5172
>
> McMaster-Carr Rubber isolator mounts:
>
> https://www.mcmaster.com/#rubber-isolators/=1alwgb6
>
> I used a metric size for the metal Azure pump and don't remember the size,
> sorry.
>
> You'll have to tap the plastic pump screw holes to install rubber isolators
> therefore you'll have figger out what size to use after
> you tap them.
>
> Regards,
> Rob M.
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Rob
> Mueller
> Sent: Friday, December 8, 2017 9:58 PM
> To: gmclist@list.gmcnet.org
> Subject: [GMCnet] Ford Azure / F-150 Eco Boost Vacuum Pumps
>
> G'day,
>
> As promised here's a revised album showing both the Ford Azure and F-150
> Eco Boost Vacuum pumps.
>
> http://www.gmcmhphotos.com/photos/g7124-ford-azure-2f-f-
> 150-eco-boost-vacuum-pump.html
>
> Feel free to ask questions.
>
> I spoke to Dave Lenzi and he's still selling his kit; please contact him
> directly for info on the kit:
>
> http://www.gmcmotorhomemarketplace.com/lenzi/index.html
>
> Regards,
> Rob M.
>
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Mike
> Sent: Saturday, December 30, 2017 6:00 AM
> To: gmclist@list.gmcnet.org
> Subject: Re: [GMCnet] Ford A426C brake vacuum pump, your opinion please?
>
> Looks like I missed out on the pump advertised on the GMC Swap Meet forum,
> I responded pretty quick but someone else beat me to it.
>
> If you have one you would like to sell please let me know?
>
> There are approximately 13 options available from the GMC motorhome page (
> http://www.gmcmotorhome.com/tech/vacuum_pump/index.html)
> as well as the GM
> offered pump, are these antiquated or have they been surpassed by newer
> technology?
>
> If there is a newer / better alternative (subjective I know) available, I'm
> still open to options as I would like to fit my coach
> with some back-up system?
>
> Thanks again,
> Mike
>
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach

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