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Home » Public Forums » GMCnet » Re: [GMCnet] Different models of P-30 master cylinders?
Re: [GMCnet] Different models of P-30 master cylinders? [message #248379] Thu, 24 April 2014 15:24 Go to next message
Tom Whitton   United States
Messages: 235
Registered: February 2004
Location: Paducah, KY
Karma: 0
Senior Member
Does anyone know if the following sent to me several years ago is still true? Are there still three different models of P-30 master cylinders and, the main reason for my question, do they all take the same length pushrod? There is quite a bit of travel on my brake pedal before the brakes engage and I want to reduce the travel if possible. The brakes are new and the rear drum shoes are adjusted tight. My brakes are excellent but I don't like the travel.

(I think Emery Stora might have written this):

"The are three different P-30 master cylinders. Two of them have a 1 1/4" bore and one has a 1 5/16" bore. I used the one with the 1 5/16" bore which is for a 1972-76 Chevrolet P-30 chassis which has 15" drums and 4" wide brake shoes. Carquest 20-1583 (rebuilt) or E84058 $122.43 (new). Price from year 2000. This one has about 10% more volume (or 10% less stroke for the same volume than the 1 1/4" bore on the other P-30 models.

It will be necessary to make a pushrod that is 3 11/16" long. Use a steel rod of the same diameter as the original and round the end on a grinding wheel or adjustable pushrods can be purchased.

Since the master cylinder is longer than the original it may be necessary to grind a reinforcing rib on the inside of the front fiberglass door the clear the bail on the master cylinder or washers can be placed between the hinge and the door to make the difference."

I made my pushrod 3 11/16" long as suggested. I don't want to unbolt everything, including the vacuum pump and connections if there isn't a possibility of reducing the pedal travel. I will appreciate your thoughts.

Thanks,

Tom Whitton
26 foot updated GMC
Paducah, KY


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Re: [GMCnet] Different models of P-30 master cylinders? [message #248381 is a reply to message #248379] Thu, 24 April 2014 15:38 Go to previous messageGo to next message
James Hupy is currently offline  James Hupy   United States
Messages: 6806
Registered: May 2010
Karma: -62
Senior Member
Tom, you are going to get a bunch of answers to your question. There are
more than 3. And, to complicate it further, if you have a bolt on
reservoir, there are several different ones there also. Where your brace
that steadies the booster/MC is located will affect push rod length as
well. From the info that you gave us, one could conclude that your pushrod
might be short. Remove the MC without taking the lines loose, pull it back
far enough to place some modelers clay on the outer end of the rod, then
reassemble without putting any pressure on the pedal. Then remove it again
and check the thickness of the clay. You should have 1/16" or so. Any more,
you need a longer rod. This is a S.W.A.G. method but is FAIRLY ACCURATE if
you are carefull. As with all things GMC, your results may vary.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Apr 24, 2014 1:25 PM, "Tom Whitton" <tomwhitton@bellsouth.net> wrote:

> Does anyone know if the following sent to me several years ago is still
> true? Are there still three different models of P-30 master cylinders and,
> the main reason for my question, do they all take the same length pushrod?
> There is quite a bit of travel on my brake pedal before the brakes engage
> and I want to reduce the travel if possible. The brakes are new and the
> rear drum shoes are adjusted tight. My brakes are excellent but I don't
> like the travel.
>
> (I think Emery Stora might have written this):
>
> "The are three different P-30 master cylinders. Two of them have a 1 1/4"
> bore and one has a 1 5/16" bore. I used the one with the 1 5/16" bore
> which is for a 1972-76 Chevrolet P-30 chassis which has 15" drums and 4"
> wide brake shoes. Carquest 20-1583 (rebuilt) or E84058 $122.43 (new).
> Price from year 2000. This one has about 10% more volume (or 10% less
> stroke for the same volume than the 1 1/4" bore on the other P-30 models.
>
> It will be necessary to make a pushrod that is 3 11/16" long. Use a steel
> rod of the same diameter as the original and round the end on a grinding
> wheel or adjustable pushrods can be purchased.
>
> Since the master cylinder is longer than the original it may be necessary
> to grind a reinforcing rib on the inside of the front fiberglass door the
> clear the bail on the master cylinder or washers can be placed between the
> hinge and the door to make the difference."
>
> I made my pushrod 3 11/16" long as suggested. I don't want to unbolt
> everything, including the vacuum pump and connections if there isn't a
> possibility of reducing the pedal travel. I will appreciate your thoughts.
>
> Thanks,
>
> Tom Whitton
> 26 foot updated GMC
> Paducah, KY
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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Re: [GMCnet] Different models of P-30 master cylinders? [message #248383 is a reply to message #248379] Thu, 24 April 2014 16:11 Go to previous messageGo to next message
Mr ERFisher is currently offline  Mr ERFisher   United States
Messages: 7117
Registered: August 2005
Karma: 2
Senior Member
Read here

http://www.machinesoflovinggrace.net/gmc/brakes/MasterCylinders.html

Erf

On Thursday, April 24, 2014, Tom Whitton <tomwhitton@bellsouth.net> wrote:

> Does anyone know if the following sent to me several years ago is still
> true? Are there still three different models of P-30 master cylinders and,
> the main reason for my question, do they all take the same length pushrod?
> There is quite a bit of travel on my brake pedal before the brakes engage
> and I want to reduce the travel if possible. The brakes are new and the
> rear drum shoes are adjusted tight. My brakes are excellent but I don't
> like the travel.
>
> (I think Emery Stora might have written this):
>
> "The are three different P-30 master cylinders. Two of them have a 1 1/4"
> bore and one has a 1 5/16" bore. I used the one with the 1 5/16" bore
> which is for a 1972-76 Chevrolet P-30 chassis which has 15" drums and 4"
> wide brake shoes. Carquest 20-1583 (rebuilt) or E84058 $122.43 (new).
> Price from year 2000. This one has about 10% more volume (or 10% less
> stroke for the same volume than the 1 1/4" bore on the other P-30 models.
>
> It will be necessary to make a pushrod that is 3 11/16" long. Use a steel
> rod of the same diameter as the original and round the end on a grinding
> wheel or adjustable pushrods can be purchased.
>
> Since the master cylinder is longer than the original it may be necessary
> to grind a reinforcing rib on the inside of the front fiberglass door the
> clear the bail on the master cylinder or washers can be placed between the
> hinge and the door to make the difference."
>
> I made my pushrod 3 11/16" long as suggested. I don't want to unbolt
> everything, including the vacuum pump and connections if there isn't a
> possibility of reducing the pedal travel. I will appreciate your thoughts.
>
> Thanks,
>
> Tom Whitton
> 26 foot updated GMC
> Paducah, KY
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>


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“Give a man a fish; you have fed him for today --- give him a URL and
-------
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http://gmcmotorhome.info/APC.html
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Re: [GMCnet] Different models of P-30 master cylinders? [message #248397 is a reply to message #248379] Thu, 24 April 2014 19:42 Go to previous message
kerry pinkerton is currently offline  kerry pinkerton   United States
Messages: 2565
Registered: July 2012
Location: Harvest, Al
Karma: 15
Senior Member
Tom, Ken Henderson did a very well done documentation on a jig for finding the correct length of any pushrod.

http://www.gmcmhphotos.com/photos/g6289-master-cylinder-pushrod-gauge.html

When I was working with Dave Lenzi at the Montgomery rally, he told me that EVERY pushrod needed to be fit individually. No two are alike.


Kerry Pinkerton - North Alabama Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L

[Updated on: Thu, 24 April 2014 19:43]

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