Home » Public Forums » GMCnet » [GMCnet] Brake Master Cylinder size
| [GMCnet] Brake Master Cylinder size [message #87384] |
Sat, 05 June 2010 12:27  |
KB
Messages: 1262
Registered: September 2009
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I posted a question about master cylinder sizes recently and got no response. From this page:
http://gmcmotorhome.info/brakes.html
it appears a smaller piston gives more available braking pressure. The stock MC had a 1 1/8" piston.
All the referenced replacements are 1 1/4". But why? Are there no appropriate 1 1/8" MC's available,
or is there some part of the calculation I'm not understanding??
I know there are folks here with a vastly greater understanding of this question than I have.
Would someone please enlighten me??
confused,
Karen
1973 23' Patience Tester
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] Brake Master Cylinder size [message #87388 is a reply to message #87384] |
Sat, 05 June 2010 13:22  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Karen,
This is probably the page you should reference for MC information:
http://users.california.com/~eagle/master%20cyl.html
<http://users.california.com/~eagle/master%20cyl.html>An equivalvent to the
OEM MC should be readily available -- I bought one at AutoZone a few years
back for <$25, off-the-shelf.
The Raybestos MC39075 is a good choice and should be <$40 today.
It is true that the smaller the MC bore, the more pressure will be supplied
to the wheel cylinders/calipers. BUT, the increased pressure comes at the
cost of more piston travel.
If you increase the size of any of the wheel actuators, then you run the
risk of exceeding the capacity of the MC. The only feasible solution to
that problem is to increase the bore of the MC -- thus all the references to
1-1/4", 1-5/16", and 34 mm MC's. Dave Mumert's calculations are offered to
enable one to make an intelligent choice among the myriad combinations
available.
HTH,
Ken H.
Americus, GA
'76 X-Birchaven
www.gmcwipersetc.com
On Sat, Jun 5, 2010 at 1:27 PM, KB <kab7@sonic.net> wrote:
> I posted a question about master cylinder sizes recently and got no
> response. From this page:
> http://gmcmotorhome.info/brakes.html
> it appears a smaller piston gives more available braking pressure. The
> stock MC had a 1 1/8" piston.
> All the referenced replacements are 1 1/4". But why? Are there no
> appropriate 1 1/8" MC's available,
> or is there some part of the calculation I'm not understanding??
> ...
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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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| Re: [GMCnet] Brake Master Cylinder size [message #87390 is a reply to message #87384] |
Sat, 05 June 2010 13:32 |
KB
Messages: 1262
Registered: September 2009
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> Many of us have put disk brakes on the rear and found that the 1-1/8"
> master cylinder did not have adequate volume for the calipers on the
> rear so we switched to master cylinders that had larger pistons.
>
> The original master cylinders with 1-1/8" should work fine for you
> and, as you have pointed out, it gives more pressure.
Thanks Emery.
Anyone have ideas for a 1 1/8" MC replacement that would work???
I saw that Corvette's use that size, but I've no idea if those would work in a GMC.
All the part numbers I've found so far in the GMC exchange are for 1 1/4".
thanks,
Karen
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] Brake Master Cylinder size [message #87439 is a reply to message #87390] |
Sat, 05 June 2010 18:56 |
emerystora
Messages: 4442
Registered: January 2004
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On Jun 5, 2010, at 12:32 PM, KB wrote:
>
>
> Anyone have ideas for a 1 1/8" MC replacement that would work???
> I saw that Corvette's use that size, but I've no idea if those would work in a GMC.
> All the part numbers I've found so far in the GMC exchange are for 1 1/4".
>
>
> thanks,
> Karen
If you are talking about the GMCMI club Parts Interchange I believe there are about a dozen listed there that are replacements for the original which has a 1-1/8" piston. This includes NAPA, Carquest, Autozone, Raybestos, EIS and some others.
Emery Stora
77 Kingsley
Santa Fe, NM
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| Re: [GMCnet] Brake Master Cylinder size [message #87539 is a reply to message #87384] |
Sun, 06 June 2010 13:52 |
KB
Messages: 1262
Registered: September 2009
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> If you are talking about the GMCMI club Parts Interchange I believe there are about a dozen listed there that are replacements for the original which has a 1-1/8" piston.
You're right, my bad. I'd checked a few of them and found they were 1.25", but didn't check them all.
The ones I saw referenced on the forum were always 1.25" if I recall correctly. Thanks for the pointer.
Now I have to wonder: how far can you go with the OEM Master Cylinder size? The best bang for the buck based on
calculations seems to be OEM 1 1/8" MC, larger 1 1/16" center wheel cylinders, OEM 15/16" rear wheel cylinders,
and larger 80mm front calipers. Not sure if that's doable with the OEM MC though. Has anybody tried this?
Did it work??
thanks,
Karen
1973 23' Patience Tester, now with functional fuel level senders
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] Brake Master Cylinder size [message #87602 is a reply to message #87539] |
Mon, 07 June 2010 06:48 |
 |
Tangerine
Messages: 192
Registered: February 2004
Location: Livonia, MI
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Karen
I have this very set up on my coach. I really like the way it stops now,
compared to before. I have had no problem with low pedal or anything else.
Gary W. Mills (Livonia MI)
. ___________
./_][__][] []| 1974 GMC M/H
.*O-------OO-* Painted Desert
"Tangerine Dream" W/New Frame
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of KB
Sent: Sunday, June 06, 2010 2:53 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Brake Master Cylinder size
> If you are talking about the GMCMI club Parts Interchange I believe there
are about a dozen listed there that are replacements for the original which
has a 1-1/8" piston.
You're right, my bad. I'd checked a few of them and found they were 1.25",
but didn't check them all.
The ones I saw referenced on the forum were always 1.25" if I recall
correctly. Thanks for the pointer.
Now I have to wonder: how far can you go with the OEM Master Cylinder size?
The best bang for the buck based on
calculations seems to be OEM 1 1/8" MC, larger 1 1/16" center wheel
cylinders, OEM 15/16" rear wheel cylinders,
and larger 80mm front calipers. Not sure if that's doable with the OEM MC
though. Has anybody tried this?
Did it work??
thanks,
Karen
1973 23' Patience Tester, now with functional fuel level senders
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
1974 GMC 260
Tangerine Dream
Livonia Michigan
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| Re: [GMCnet] Brake Master Cylinder size [message #87608 is a reply to message #87602] |
Mon, 07 June 2010 08:15 |
gmcrv1
Messages: 839
Registered: August 2007
Location: Memphis
Karma: -1
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Gary,
Excuse me, but I may have missed a post. Would you mind repeating the brake
system set up you have on your coach? MC diameter, front disks, middle WC
and rear WC.
Thank You,
Tom Eckert
73 Glacier
Oakland, TN
On Mon, Jun 7, 2010 at 6:48 AM, Gary W. Mills <daveel@ix.netcom.com> wrote:
> Karen
> I have this very set up on my coach. I really like the way it stops now,
> compared to before. I have had no problem with low pedal or anything else.
> Gary W. Mills (Livonia MI)
> . ___________
> ./_][__][] []| 1974 GMC M/H
> .*O-------OO-* Painted Desert
> "Tangerine Dream" W/New Frame
>
>
>
> -----Original Message-----
> From: gmclist-bounces@temp.gmcnet.org
> [mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of KB
> Sent: Sunday, June 06, 2010 2:53 PM
> To: gmclist@temp.gmcnet.org
> Subject: Re: [GMCnet] Brake Master Cylinder size
>
>
>
> > If you are talking about the GMCMI club Parts Interchange I believe there
> are about a dozen listed there that are replacements for the original which
> has a 1-1/8" piston.
>
>
> You're right, my bad. I'd checked a few of them and found they were 1.25",
> but didn't check them all.
> The ones I saw referenced on the forum were always 1.25" if I recall
> correctly. Thanks for the pointer.
>
> Now I have to wonder: how far can you go with the OEM Master Cylinder size?
> The best bang for the buck based on
> calculations seems to be OEM 1 1/8" MC, larger 1 1/16" center wheel
> cylinders, OEM 15/16" rear wheel cylinders,
> and larger 80mm front calipers. Not sure if that's doable with the OEM MC
> though. Has anybody tried this?
> Did it work??
>
> thanks,
> Karen
> 1973 23' Patience Tester, now with functional fuel level senders
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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| Re: [GMCnet] Brake Master Cylinder size [message #87610 is a reply to message #87608] |
Mon, 07 June 2010 08:33 |
|
Tangerine
Messages: 192
Registered: February 2004
Location: Livonia, MI
Karma: 0
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Senior Member |
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Tom
I was replying to Karen. I have OEM 1 1/8" MC, larger 1 1/16" center wheel
cylinders, OEM 15/16" rear wheel cylinders, and larger 80mm front calipers.
Gary W. Mills (Livonia MI)
. ___________
./_][__][] []| 1974 GMC M/H
.*O-------OO-* Painted Desert
"Tangerine Dream" W/New Frame
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of Tom Eckert
Sent: Monday, June 07, 2010 9:15 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Brake Master Cylinder size
Gary,
Excuse me, but I may have missed a post. Would you mind repeating the brake
system set up you have on your coach? MC diameter, front disks, middle WC
and rear WC.
Thank You,
Tom Eckert
73 Glacier
Oakland, TN
On Mon, Jun 7, 2010 at 6:48 AM, Gary W. Mills <daveel@ix.netcom.com> wrote:
> Karen
> I have this very set up on my coach. I really like the way it stops now,
> compared to before. I have had no problem with low pedal or anything else.
> Gary W. Mills (Livonia MI)
> . ___________
> ./_][__][] []| 1974 GMC M/H
> .*O-------OO-* Painted Desert
> "Tangerine Dream" W/New Frame
>
>
>
> -----Original Message-----
> From: gmclist-bounces@temp.gmcnet.org
> [mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of KB
> Sent: Sunday, June 06, 2010 2:53 PM
> To: gmclist@temp.gmcnet.org
> Subject: Re: [GMCnet] Brake Master Cylinder size
>
>
>
> > If you are talking about the GMCMI club Parts Interchange I believe
there
> are about a dozen listed there that are replacements for the original
which
> has a 1-1/8" piston.
>
>
> You're right, my bad. I'd checked a few of them and found they were
1.25",
> but didn't check them all.
> The ones I saw referenced on the forum were always 1.25" if I recall
> correctly. Thanks for the pointer.
>
> Now I have to wonder: how far can you go with the OEM Master Cylinder
size?
> The best bang for the buck based on
> calculations seems to be OEM 1 1/8" MC, larger 1 1/16" center wheel
> cylinders, OEM 15/16" rear wheel cylinders,
> and larger 80mm front calipers. Not sure if that's doable with the OEM MC
> though. Has anybody tried this?
> Did it work??
>
> thanks,
> Karen
> 1973 23' Patience Tester, now with functional fuel level senders
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
1974 GMC 260
Tangerine Dream
Livonia Michigan
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| Re: [GMCnet] Brake Master Cylinder size [message #87612 is a reply to message #87610] |
Mon, 07 June 2010 08:54 |
gmcrv1
Messages: 839
Registered: August 2007
Location: Memphis
Karma: -1
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Senior Member |
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Gary,
I knew you were addressing Karen, but I have a completely original setup on
the Glacier and for now want to make that the best it can be without going
overboard. Looks like I can go with the 1 1/16" center as the first upgrade
and follow that with the 80mm front calipers.
This coach is new to me and I haven't ventured into the brakes and
suspension yet...
Thanks Again,
Tom
73 Glacier
On Mon, Jun 7, 2010 at 8:33 AM, Gary W. Mills <daveel@ix.netcom.com> wrote:
> Tom
> I was replying to Karen. I have OEM 1 1/8" MC, larger 1 1/16" center wheel
> cylinders, OEM 15/16" rear wheel cylinders, and larger 80mm front calipers.
> Gary W. Mills (Livonia MI)
> . ___________
> ./_][__][] []| 1974 GMC M/H
> .*O-------OO-* Painted Desert
> "Tangerine Dream" W/New Frame
>
>
>
> -----Original Message-----
> From: gmclist-bounces@temp.gmcnet.org
> [mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of Tom Eckert
> Sent: Monday, June 07, 2010 9:15 AM
> To: gmclist@temp.gmcnet.org
> Subject: Re: [GMCnet] Brake Master Cylinder size
>
>
> Gary,
>
> Excuse me, but I may have missed a post. Would you mind repeating the
> brake
> system set up you have on your coach? MC diameter, front disks, middle WC
> and rear WC.
>
> Thank You,
>
> Tom Eckert
> 73 Glacier
> Oakland, TN
>
> On Mon, Jun 7, 2010 at 6:48 AM, Gary W. Mills <daveel@ix.netcom.com>
> wrote:
>
> > Karen
> > I have this very set up on my coach. I really like the way it stops now,
> > compared to before. I have had no problem with low pedal or anything
> else.
> > Gary W. Mills (Livonia MI)
> > . ___________
> > ./_][__][] []| 1974 GMC M/H
> > .*O-------OO-* Painted Desert
> > "Tangerine Dream" W/New Frame
> >
> >
> >
> > -----Original Message-----
> > From: gmclist-bounces@temp.gmcnet.org
> > [mailto:gmclist-bounces@temp.gmcnet.org]On Behalf Of KB
> > Sent: Sunday, June 06, 2010 2:53 PM
> > To: gmclist@temp.gmcnet.org
> > Subject: Re: [GMCnet] Brake Master Cylinder size
> >
> >
> >
> > > If you are talking about the GMCMI club Parts Interchange I believe
> there
> > are about a dozen listed there that are replacements for the original
> which
> > has a 1-1/8" piston.
> >
> >
> > You're right, my bad. I'd checked a few of them and found they were
> 1.25",
> > but didn't check them all.
> > The ones I saw referenced on the forum were always 1.25" if I recall
> > correctly. Thanks for the pointer.
> >
> > Now I have to wonder: how far can you go with the OEM Master Cylinder
> size?
> > The best bang for the buck based on
> > calculations seems to be OEM 1 1/8" MC, larger 1 1/16" center wheel
> > cylinders, OEM 15/16" rear wheel cylinders,
> > and larger 80mm front calipers. Not sure if that's doable with the OEM MC
> > though. Has anybody tried this?
> > Did it work??
> >
> > thanks,
> > Karen
> > 1973 23' Patience Tester, now with functional fuel level senders
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
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| Re: [GMCnet] Brake Master cylinder [message #355470 is a reply to message #87384] |
Sun, 07 June 2020 14:26 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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You may want to make one of these to measure the required pushrod length:
http://www.gmcmhphotos.com/photos/g6289-master-cylinder-pushrod-gauge.html
Ken H.
On Sun, Jun 7, 2020 at 3:23 PM Emery Stora via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Either of those will do. Just measure the thickness of the bolt flanges.
> If the new ones are thicker it will require a longer push rod. If thinner
> you’ll need a shorter push rod.
>
> Emery Stora
>
>> On Jun 7, 2020, at 12:47 PM, Jon Roche via Gmclist gmclist@list.gmcnet.org> wrote:
>>
>> Just installed a dorman one last week in a gmc. Fit perfectly and the
> casting was NOT too thick.
>>
>>
>> I had went in and asked for a “centric” as billy recently installed:
>>
>>
> https://www.amazon.com/Centric-Parts-130-66008-Master-Cylinder/dp/B000C0340G?fbclid=IwAR0iEdPS7DqV5se12dE7SPNZFjkx_Dsaf9KJQ7hCn6s9rwm2YnHUhtbT9I4
>>
>> But locally was not available and they had the Dorman one in stock.
>>
>>
>> --
>> Jon Roche
>> 75 palm beach
>> EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
>> St. Cloud, MN
>> http://lqqkatjon.blogspot.com/
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
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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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| Re: [GMCnet] Brake Master Cylinder size [message #355505 is a reply to message #87384] |
Mon, 08 June 2020 15:17 |
lqqkatjon
Messages: 2324
Registered: October 2010
Location: St. Cloud, MN
Karma: 5
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Senior Member |
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seems like I have been tossing brake parts at my coach for years.... 9 years ago, I installed a sensitized booster bought at the time from the GMC Coop. few years ago, I went and replaced all rear brake shoes, springs and cylinders and up'd the size of the mid axle wheel cylinder.
Last fall, I changed out stock calipers up front for the 80mm calipers, and installed perfomance friction brake pads. We used a special tool to really prep up the rotors, and those Pads "bedded" in real fast and really well. I bet I had less then $150 in doing that change. (I had probably 8 year old stock front brakes).
So with a 9 year old sensitized brake booster, a stock master cylinder, 80mm front calipers running performance friction pads, the larger mid wheel cylinder and stock rear brakes. I have no concern at all about stopping. I have been driving my coach almost as a daily driver this spring, and stop/go traffic, around town and it stops just as well as my honda crv.
I have recently driven some coaches that have no "sensitized" booster, and have an old stock one in the coach. And they stop fine, but I really would not want to drive them around town like I do my coach. and would not want to ever try to stop them without wearing shoes.
so in my opinion, running a sensitized booster, stock master cylinder, 80mm brakes(only if you need to change bad 70mm), and a mid axle larger cylinder(again- only if you are changing brakes there). makes for a really pleasant driving/stopping experience.
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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| Re: [GMCnet] Brake Master Cylinder size [message #355506 is a reply to message #355505] |
Mon, 08 June 2020 15:47 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
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I know that it seem like GMC brakes need a lot of maintenance. To be
honest, they do. It is a heavy vehicle, and old. Drums, linings, rotors and
pads are all items that, if they do their job well, wear as we use them.
Fact of life. But, they fail when the coach sits without being used. But
there, it is caliper seals, wheel cylinders, master cylinders, distribution
valves, brake lines. It is all part of owning a motorhome. You cannot fix
it once, and forget about it. Brake fluid loves to absorb water from the
air. Periodic fluid changes are mandatory, not just suggestions. I don't
like it much, either.
Jim Hupy
Salem, Oregon
On Mon, Jun 8, 2020, 1:17 PM Jon Roche via Gmclist
wrote:
> seems like I have been tossing brake parts at my coach for years.... 9
> years ago, I installed a sensitized booster bought at the time from the GMC
> Coop. few years ago, I went and replaced all rear brake shoes, springs
> and cylinders and up'd the size of the mid axle wheel cylinder.
>
> Last fall, I changed out stock calipers up front for the 80mm calipers,
> and installed perfomance friction brake pads. We used a special tool to
> really prep up the rotors, and those Pads "bedded" in real fast and really
> well. I bet I had less then $150 in doing that change. (I had probably 8
> year old stock front brakes).
>
> So with a 9 year old sensitized brake booster, a stock master cylinder,
> 80mm front calipers running performance friction pads, the larger mid wheel
> cylinder and stock rear brakes. I have no concern at all about stopping.
> I have been driving my coach almost as a daily driver this spring, and
> stop/go traffic, around town and it stops just as well as my honda crv.
>
>
> I have recently driven some coaches that have no "sensitized" booster, and
> have an old stock one in the coach. And they stop fine, but I really would
> not want to drive them around town like I do my coach. and would not
> want to ever try to stop them without wearing shoes.
>
> so in my opinion, running a sensitized booster, stock master cylinder,
> 80mm brakes(only if you need to change bad 70mm), and a mid axle larger
> cylinder(again- only if you are changing brakes there). makes for a
> really pleasant driving/stopping experience.
>
>
> --
> Jon Roche
> 75 palm beach
> EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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