Home » Public Forums » GMCnet » P30 front and rear lines were not reversed
| P30 front and rear lines were not reversed [message #168623] |
Sun, 06 May 2012 09:31  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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I changed my rear drums to disks. In the process I discovered the steel lines at the back needed to be replaced too. I decided to let my local Fleet garage change the lines and change the master cylinder to the P30. I had ordered a brake line kit at the same time as the disk kit just in case.
Now I have noticed that they did not reverse the front and rear ports of the MC as required with the P30 MC. They re-flared the original lines and used an adapter on the rear port (I had asked them to change them only if they didn't look good). My brakes work great now so I'm wondering if this makes any difference?? After all the OEM MC was built front brakes on the rear of the MC.
The brake warning light does not come on so the proportioning valve must be happy.
What say yea?
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: [GMCnet] P30 front and rear lines were not reversed [message #168626 is a reply to message #168625] |
Sun, 06 May 2012 10:13  |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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Senior Member |
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On Sun, May 6, 2012 at 8:06 AM, Bruce Hislop <bruce@perthcomm.com> wrote:
>
>
> I read everything I could find and all indications were that it needed to
> be changed to get the required displacement.
>
> read here
http://gmcmotorhome.info/brakes.html#calc
and
here
http://www.bdub.net/branscombe/
you loose about 1000# clamping pressure
good luck
gene
> I did have the OEM MC still in place on the way to the garage and the
> pedal was way down from the drums.
>
> We are going out now for a spring shakeout to Lake Huron, about an hour
> away. If you never see me post again then you'll know its important to
> switch the lines!
> :roll:
>
> Its amazing how much the piston area changes from 1.125 to 1.25, about 25%
> difference.
>
> --
> Bruce Hislop
> ON Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
> Hubler 1 ton front end
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> My Staff says I never listen to them, or something like that
> _______________________________________________
> GMCnet mailing list
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--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
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Alternator Protection Cable
http://gmcmotorhome.info/APC.html
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| Re: P30 front and rear lines were not reversed [message #168652 is a reply to message #168640] |
Sun, 06 May 2012 16:37 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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| rf_burns wrote on Sun, 06 May 2012 13:50 |
Well we made it back, brakes still working great!
Maybe a bit more nose-dive than before. or maybe no.. CRS!
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Glad you made it. Judging by some of the earlier posts, you should be dead 
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: P30 front and rear lines were not reversed [message #168740 is a reply to message #168623] |
Mon, 07 May 2012 10:55 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Senior Member |
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| rf_burns wrote on Sun, 06 May 2012 07:31 |
Now I have noticed that they did not reverse the front and rear ports of the MC as required with the P30 MC. They re-flared the original lines and used an adapter on the rear port (I had asked them to change them only if they didn't look good). My brakes work great now so I'm wondering if this makes any difference?? After all the OEM MC was built front brakes on the rear of the MC.
What say yea?
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I don't like the sound of it. When we put on rear disks, we switched the lines. Pain to sort of bend them around. I thought the front and rear piston diameter is different in the master cylinder and I figure that is because a smart guy knew something I don't know. But that is just me.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] P30 front and rear lines were not reversed [message #168742 is a reply to message #168732] |
Mon, 07 May 2012 11:22 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Bruce,
Don't worry about it. Here's an album of testing I did on a P-30 some
years ago. I don't remember the conclusions I reached, but the primary and
secondary pistons' diameters are the same which I figure is the most
important parameter. Just watch the fluid levels because the rears, with
all that piston area, will require more fluid than the fronts as the pads
wear.
http://www.gmcmhphotos.com/photos/g3457-p-30-master-cylinder-testing.html
or
http://goo.gl/HkPkq
Ken H.
On Mon, May 7, 2012 at 10:57 AM, Bruce Hislop wrote:
>
>
> I'm just wondering if there is some critical reason for the change in
> ports.
>
> Does the rear port have more displacement for the additional calipers? or
> was it just to make things make more sense.. front brakes to front port..
> rears to rear... (Hmmm not likely)
>
> --
>
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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] P30 front and rear lines were not reversed [message #168749 is a reply to message #168748] |
Mon, 07 May 2012 11:56 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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Bruce
Quit worrying about which port the front and rear go to.
This master cylinder has a 1/2x20 threaded port for the front chamber and a 9/16x18 threaded port for the back chamber.
NOTE: the original GMC master cylinder had the 1/2 in the back and the 9/16 in the front. So it is necessary to move the 9/16" (rear brake line) to the rear chamber and the 1/2" (front brake line) to the front chamber. This is not a problem as this master cylinder has two chambers that are of equal size - unlike the original one that has a smaller front chamber that went to the rear brakes.
So it really doesn't matter which port the front or the rear are to with your new master cylinder. It sounds as if your installer went to a lot of trouble and expense (which you paid for) by making up adapters and reflaring the fittings to keep the rear brakes to the front chamber
Emery Stora
On May 7, 2012, at 10:40 AM, Bruce Hislop wrote:
>
>
> George,
> I down-loaded the spreadsheet version of the brake calculator and did some modifications to it.
>
> I added a 75mm caliper since that was on my front before One Ton and its what my one ton kit has.
>
> I also added a calculator to allow you to put different sizes of calipers front, mid and rear and it will tell you the master cylinder travel.
>
> I had a conversation with Albert Branscombe about a month ago and he said 1.25 was an OEM MC, maybe 1.125 was an early version? Anyway when I saw how far my pedal went down I got to thinking I must have a 1.125 MC, or maybe my MC has a fault. Anyway I had the P30 installed.
>
> In the calculator it shows no difference in the front/back ports as far as the calculations go.
>
> FYI the difference between 1.25 and P30 is about 1/4" in travel.
>
> I can send my spreadsheet for uploading.. I hope I haven't stepped on anyone's copywrite.
>
> --
> Bruce Hislop
> ON Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
> Hubler 1 ton front end
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> My Staff says I never listen to them, or something like that
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] P30 front and rear lines were not reversed [message #168776 is a reply to message #168749] |
Mon, 07 May 2012 16:51 |
GMC_LES
Messages: 569
Registered: October 2009
Location: Montreal
Karma: 0
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Senior Member |
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The most important part is to ensure that the port with the largest reservoir is connected to the circuit with the highest displacement. With 6 wheel disc brakes, that would be the rear. The P30 master cylinder has equal sized reservoirs that appear to be slightly larger than the largest reservoir in the OEM unit, so line orientation isn't important.
The reason for connecting the larger reservoir to the circuit with the highest displacement is to ensure you have sufficient fluid supply to compensate for brake pad wear. Four calipers on one circuit will take up a fair quantity of fluid as the caliper pistons extend to compensate for brake pad wear.
I feel even the P30's individual reservoir capacities are likely marginal for supplying four rear calipers of 75 & 80mm, so the most important thing is to keep an eye on your brake fluid levels as the brakes wear. Other than that minor concern, you should be good for years of trouble free service.
JimK mentioned drilling a hole mid-way down in the partition between the reservoirs of a master cylinder as a method of increasing fluid supply to either circuit. He didn't specifically suggest it for the P30 master cylinder, but I don't see why it wouldn't help should one be concerned about fluid supply.
Your mileage may vary !! ;)
Les Burt
Montreal
On 2012-05-07, at 12:56 PM, Emery Stora <emerystora@mac.com> wrote:
> Bruce
>
> Quit worrying about which port the front and rear go to.
>
> This master cylinder has a 1/2x20 threaded port for the front chamber and a 9/16x18 threaded port for the back chamber.
> NOTE: the original GMC master cylinder had the 1/2 in the back and the 9/16 in the front. So it is necessary to move the 9/16" (rear brake line) to the rear chamber and the 1/2" (front brake line) to the front chamber. This is not a problem as this master cylinder has two chambers that are of equal size - unlike the original one that has a smaller front chamber that went to the rear brakes.
>
> So it really doesn't matter which port the front or the rear are to with your new master cylinder. It sounds as if your installer went to a lot of trouble and expense (which you paid for) by making up adapters and reflaring the fittings to keep the rear brakes to the front chamber
>
> Emery Stora
>
> On May 7, 2012, at 10:40 AM, Bruce Hislop wrote:
>
>>
>>
>> George,
>> I down-loaded the spreadsheet version of the brake calculator and did some modifications to it.
>>
>> I added a 75mm caliper since that was on my front before One Ton and its what my one ton kit has.
>>
>> I also added a calculator to allow you to put different sizes of calipers front, mid and rear and it will tell you the master cylinder travel.
>>
>> I had a conversation with Albert Branscombe about a month ago and he said 1.25 was an OEM MC, maybe 1.125 was an early version? Anyway when I saw how far my pedal went down I got to thinking I must have a 1.125 MC, or maybe my MC has a fault. Anyway I had the P30 installed.
>>
>> In the calculator it shows no difference in the front/back ports as far as the calculations go.
>>
>> FYI the difference between 1.25 and P30 is about 1/4" in travel.
>>
>> I can send my spreadsheet for uploading.. I hope I haven't stepped on anyone's copywrite.
>>
>> --
>> Bruce Hislop
>> ON Canada
>> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
>> Hubler 1 ton front end
>> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
>> My Staff says I never listen to them, or something like that
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
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Les Burt
Montreal
1975 Eleganza 26ft
A work in Progress
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| Re: [GMCnet] P30 front and rear lines were not reversed [message #168786 is a reply to message #168776] |
Mon, 07 May 2012 17:50 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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Senior Member |
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| Les Burt wrote on Mon, 07 May 2012 14:51 |
...
JimK mentioned drilling a hole mid-way down in the partition between the reservoirs of a master cylinder as a method of increasing fluid supply to either circuit. He didn't specifically suggest it for the P30 master cylinder, ...
|
Just pointing something out:
The "mid-way down" is important. You do not want them connected all the way to the bottom. You want some division just in case you have a bad leak in one system. (Front or rear) With the lower half of the reservoirs still separate, you SHOULD still have brakes on the system with no leak. Just not much fluid in reserve.
I seem to remember a slit cut half way down in the divider between the two sides of a master cylinder reservoir.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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