Home » Public Forums » GMCnet » Re: [GMCnet] Pressure bleeding question
| Re: [GMCnet] Pressure bleeding question [message #257599] |
Sun, 03 August 2014 10:22  |
Tom Whitton
Messages: 235
Registered: February 2004
Location: Paducah, KY
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I pressure bled my brakes but did not by-pass the combination valve as recommended in the maintenance manual. The process went well.
Why is by-passing the combination valve recommended and can not doing it damage something in the system?
Appreciate your insight.
Tom Whitton
26 foot updated GMC
Paducah, KY
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| Re: [GMCnet] Pressure bleeding question [message #257601 is a reply to message #257599] |
Sun, 03 August 2014 10:39  |
Ken Henderson
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Registered: March 2004
Location: Americus, GA
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Tom,
Since it SHOULD require 135 psi from the MC before the combination valve's
metering section (what you open by depressing the little pin at the front)
allows pressure to the front calipers, you should not be able to bleed the
front brake without depressing the pin. But that assumes that the
combination valve is not corroded, corrupted, or full of rust & other crud.
Not a very good bet if it's original.
You can't hurt anything by not bypassing the metering valve -- except the
un-flushed front calipers. :-)
Of course, if you've got disc brakes in the rear, that valve should have
been eliminated anyway.
http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
Ken H.
On Sun, Aug 3, 2014 at 11:22 AM, Tom Whitton
wrote:
> I pressure bled my brakes but did not by-pass the combination valve as
> recommended in the maintenance manual. The process went well.
>
> Why is by-passing the combination valve recommended and can not doing it
> damage something in the system?
>
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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] Pressure bleeding question [message #257603 is a reply to message #257601] |
Sun, 03 August 2014 11:08 |
thorndike
Messages: 406
Registered: January 2011
Location: Conifer, Colorado
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Ken,
When I installed my 6 wheel disc brakes JimK didn't say anything about not needing the combination valve. In fact, while troubleshooting the installation, he sent me a replacement. So I am a bit confused. One of the things I DID learn from your post though, was the need to depress the button to bleed the front brakes. This may be the cause of why my brakes aren't as good as they should be. Thanks for that!
Bob
Ken Henderson wrote on Sun, 03 August 2014 11:39Tom,
Since it SHOULD require 135 psi from the MC before the combination valve's
metering section (what you open by depressing the little pin at the front)
allows pressure to the front calipers, you should not be able to bleed the
front brake without depressing the pin. But that assumes that the
combination valve is not corroded, corrupted, or full of rust & other crud.
Not a very good bet if it's original.
You can't hurt anything by not bypassing the metering valve -- except the
un-flushed front calipers. 
Of course, if you've got disc brakes in the rear, that valve should have
been eliminated anyway.
http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
Ken H.
On Sun, Aug 3, 2014 at 11:22 AM, Tom Whitton
wrote:
> I pressure bled my brakes but did not by-pass the combination valve as
> recommended in the maintenance manual. The process went well.
>
> Why is by-passing the combination valve recommended and can not doing it
> damage something in the system?
>
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Robert Peesel
1976 Royale 26'
Side Dry Bath
Conifer, Colorado
[Updated on: Sun, 03 August 2014 11:09] Report message to a moderator |
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| Re: [GMCnet] Pressure bleeding question [message #257610 is a reply to message #257603] |
Sun, 03 August 2014 11:33 |
James Hupy
Messages: 6806
Registered: May 2010
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The combination valve's main purpose in an all disc brake system is to
serve as an isolation valve to separate the front brakes from the rear
brakes. The whole purpose is to provide redundancy should you experience
front or rear fluid loss with subsequent failure of braking effort. The
internal spring loaded plunger is not needed in AN ALL DISC SYSTEM. It is
needed in a drum/disc system to prevent the front discs from applying
before the rear drums have begun to work. More to it than that, but that is
the simple story. If the valve that Jim K. provided to you does not have a
plunger, it should only be used if you have all disc brakes.
Jim Hupy
Salem, Or.
78 GMC ROYALE 403
On Aug 3, 2014 9:08 AM, "Robert Peesel" wrote:
> Ken,
>
> When I installed my 6 wheel disc brakes JimK didn't say anything about not
> needing the combination valve. In fact, while troubleshooting the
> installation, he sent me a replacement. So I am a bit confused.
>
> Bob
>
> Ken Henderson wrote on Sun, 03 August 2014 11:39
>> Tom,
>>
>> Since it SHOULD require 135 psi from the MC before the combination
> valve's
>> metering section (what you open by depressing the little pin at the
> front)
>> allows pressure to the front calipers, you should not be able to bleed
> the
>> front brake without depressing the pin. But that assumes that the
>> combination valve is not corroded, corrupted, or full of rust & other
> crud.
>> Not a very good bet if it's original.
>>
>> You can't hurt anything by not bypassing the metering valve -- except the
>> un-flushed front calipers. :)
>>
>> Of course, if you've got disc brakes in the rear, that valve should have
>> been eliminated anyway.
>>
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
>>
>> Ken H.
>>
>> On Sun, Aug 3, 2014 at 11:22 AM, Tom Whitton
>> wrote:
>>
>>> I pressure bled my brakes but did not by-pass the combination valve as
>>> recommended in the maintenance manual. The process went well.
>>>
>>> Why is by-passing the combination valve recommended and can not doing
> it
>>> damage something in the system?
>>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
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>
>
> --
> Robert Peesel
>
> 1976 Royale 26'
>
> Side Dry Bath
>
> Sterling, Va
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| Re: [GMCnet] Pressure bleeding question [message #257621 is a reply to message #257599] |
Sun, 03 August 2014 12:45 |
Tom Whitton
Messages: 235
Registered: February 2004
Location: Paducah, KY
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Ken,
Thanks, all six wheels seemed to bleed very well but that may not be the case. I'll re-do and depress the pin this time. As far as I know, the combination valve is 41-years old, condition unknown. Still running drums in rear with new shoes. Running good yellow sticky pads in front. Brakes work reasonably well but, sadly, not as well as a modern vehicle. I thought I had paid for everything new but the body and frame. It's good to know the fun isn't over. :-)
Tom Whitton
26 foot updated GMC
Paducah, KY
Tom,
Since it SHOULD require 135 psi from the MC before the combination valve's
metering section (what you open by depressing the little pin at the front)
allows pressure to the front calipers, you should not be able to bleed the
front brake without depressing the pin. But that assumes that the
combination valve is not corroded, corrupted, or full of rust & other crud.
Not a very good bet if it's original.
You can't hurt anything by not bypassing the metering valve -- except the
un-flushed front calipers. :-)
Of course, if you've got disc brakes in the rear, that valve should have
been eliminated anyway.
http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
Ken H.
On Sun, Aug 3, 2014 at 11:22 AM, Tom Whitton
wrote:
> I pressure bled my brakes but did not by-pass the combination valve as
> recommended in the maintenance manual. The process went well.
>
> Why is by-passing the combination valve recommended and can not doing it
> damage something in the system?
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| Re: [GMCnet] Pressure bleeding question [message #257663 is a reply to message #257599] |
Sun, 03 August 2014 17:57 |
lqqkatjon
Messages: 2324
Registered: October 2010
Location: St. Cloud, MN
Karma: 5
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I have bled the front and rear brakes a few different times using the hupy pressure bleeder. I have never pressed my brand new brass valve.
I will say the fluid does not come screaming out any of the bleeders, but fluid does come out, and it takes fluid as i go around each bleeder.
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] Pressure bleeding question [message #257665 is a reply to message #257663] |
Sun, 03 August 2014 18:23 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Jon,
Something just occurred to me: Does your new combination valve have
the pin on the front? Seems to me it's possible later designs may not work
just like the GMC OEM unit. I can imagine a metering valve passing a
limited flow before going onto its pressure limiting schedule. That would
be a convenience eliminating the need to "Press the Pin".
I have no idea whether such a feature exists; it's just a sudden WAG.
Ken H.
On Sun, Aug 3, 2014 at 6:57 PM, Jon Roche wrote:
> I have bled the front and rear brakes a few different times using the hupy
> pressure bleeder. I have never pressed my brand new brass valve.
>
> I will say the fluid does not come screaming out any of the bleeders, but
> fluid does come out, and it takes fluid as i go around each bleeder.
>
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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] Pressure bleeding question [message #257677 is a reply to message #257665] |
Sun, 03 August 2014 19:41 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Ken, there you go giving away one of my hard earned secrets. Most of the
combo valves I have tested let a small pressure like 10 psi go unchecked.
That is why my instructions direct the user to only pump up that much. It
has proven to be entirely effective at bleeding, and there is less chance
of adding pressurized air into the system. Dang.
JimHupy
On Aug 3, 2014 4:24 PM, "Ken Henderson" wrote:
> Jon,
>
> Something just occurred to me: Does your new combination valve have
> the pin on the front? Seems to me it's possible later designs may not work
> just like the GMC OEM unit. I can imagine a metering valve passing a
> limited flow before going onto its pressure limiting schedule. That would
> be a convenience eliminating the need to "Press the Pin".
>
> I have no idea whether such a feature exists; it's just a sudden WAG.
>
> Ken H.
>
>
> On Sun, Aug 3, 2014 at 6:57 PM, Jon Roche wrote:
>
>> I have bled the front and rear brakes a few different times using the
> hupy
>> pressure bleeder. I have never pressed my brand new brass valve.
>>
>> I will say the fluid does not come screaming out any of the bleeders,
> but
>> fluid does come out, and it takes fluid as i go around each bleeder.
>>
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| Re: [GMCnet] Pressure bleeding question [message #257686 is a reply to message #257677] |
Sun, 03 August 2014 20:38 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Sorry 'bout that. "Even a blind hog finds an acorn now & then."
Ken H.
On Sun, Aug 3, 2014 at 8:41 PM, James Hupy wrote:
> Ken, there you go giving away one of my hard earned secrets. Most of the
> combo valves I have tested let a small pressure like 10 psi go unchecked.
> That is why my instructions direct the user to only pump up that much. It
> has proven to be entirely effective at bleeding, and there is less chance
> of adding pressurized air into the system. Dang.
> JimHupy
> On Aug 3, 2014 4:24 PM, "Ken Henderson" wrote:
>
>> Jon,
>>
>> Something just occurred to me: Does your new combination valve have
>> the pin on the front? Seems to me it's possible later designs may not
> work
>> just like the GMC OEM unit. I can imagine a metering valve passing a
>> limited flow before going onto its pressure limiting schedule. That
> would
>> be a convenience eliminating the need to "Press the Pin".
>>
>> I have no idea whether such a feature exists; it's just a sudden WAG.
>
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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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