Home » Public Forums » GMCnet » [GMCnet] Disc brake project still in failure mode
| Re: [GMCnet] Disc brake project still in failure mode [message #282523 is a reply to message #282516] |
Sun, 19 July 2015 18:48   |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Seems to me the pressure drop across the combination valve can only be
caused by the shuttle valve moving at about 500 psi and completely blocking
the rear outlet. Otherwise, the outlet pressure would eventually reach the
input pressure. Only that scenario separates the normally single closed
system into two "independent" systems.
Ken H.
On Sun, Jul 19, 2015 at 7:11 PM, A. wrote:
> Neil Fonville wrote on Sun, 19 July 2015 17:58
>> I have confirmed I have the rear port on the P30 connected to the rear
> brakes. Tested pressure again. Got 800 rising to 900 when holding on the
>> rear MC port. 500 steady at the rear output of the combination valve.
>> Did not have parts to test at the input of the valve.
>>
>> Neil
> You are within a couple shovels of paydirt. You got a 300 - 400 PSI drop
> between the master cylinder and the output of the combination valve. I don't
> know or care how that can even be possible. But you need to figure out
> which component has the evil in it - the line or the valve. Disconnect the
> line
> between the master cylinder and the combination valve and verify there
> isn't anything in it that would restrict flow. Then do the same between the
> input and output of the combination valve. Might need to replace the
> combination valve. Reassemble, bleed the rears one last time, and drive it.
> --
> '73 23' Sequoia For Sale
> '73 23' CanyonLands For Sale
> Upper Alabama
> On "marriage equality": Once you define the direction that society is
> headed as "forward", you can call it progress.
>
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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] Disc brake project still in failure mode [message #282525 is a reply to message #282523] |
Sun, 19 July 2015 18:55 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Thinking that through a bit more: If the shuttle is indeed moving and
blocking the rear outlet, the root cause is obviously remaining air in the
rear system. Otherwise there should not be a differential pressure across
the shuttle in the first place. The movement/blocking may, in fact, be a
design feature.
Ken H.
On Sun, Jul 19, 2015 at 7:48 PM, Ken Henderson
wrote:
> Seems to me the pressure drop across the combination valve can only be
> caused by the shuttle valve moving at about 500 psi and completely blocking
> the rear outlet. Otherwise, the outlet pressure would eventually reach the
> input pressure. Only that scenario separates the normally single closed
> system into two "independent" systems.
>
> Ken H.
>
>
> On Sun, Jul 19, 2015 at 7:11 PM, A. wrote:
>
>> Neil Fonville wrote on Sun, 19 July 2015 17:58
>>> I have confirmed I have the rear port on the P30 connected to the rear
>> brakes. Tested pressure again. Got 800 rising to 900 when holding on the
>>> rear MC port. 500 steady at the rear output of the combination valve.
>>> Did not have parts to test at the input of the valve.
>>>
>>> Neil
>> You are within a couple shovels of paydirt. You got a 300 - 400 PSI drop
>> between the master cylinder and the output of the combination valve. I don't
>> know or care how that can even be possible. But you need to figure out
>> which component has the evil in it - the line or the valve. Disconnect the
>> line
>> between the master cylinder and the combination valve and verify there
>> isn't anything in it that would restrict flow. Then do the same between the
>> input and output of the combination valve. Might need to replace the
>> combination valve. Reassemble, bleed the rears one last time, and drive it.
>> --
>> '73 23' Sequoia For Sale
>> '73 23' CanyonLands For Sale
>> Upper Alabama
>> On "marriage equality": Once you define the direction that society is
>> headed as "forward", you can call it progress.
>>
>> _______________________________________________
>> 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] Disc brake project still in failure mode [message #282526 is a reply to message #282523] |
Sun, 19 July 2015 18:59 |
James Hupy
Messages: 6806
Registered: May 2010
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Comes back to air in the system, compressing, allowing the spool valve to
obstruct the ports, thereby creating a pressure differential. When that
valve has equal pressure applied to both sides, it stays centered allowing
equal but separate fluid streams to go to both front and rear. If your
valve is as crude as others that I have encountered, it's possible that
there is a burr preventing the valve from centering. Just a guess, but one
worth checking out to verify.
Jim Hupy
On Jul 19, 2015 4:49 PM, "Ken Henderson" wrote:
> Seems to me the pressure drop across the combination valve can only be
> caused by the shuttle valve moving at about 500 psi and completely blocking
> the rear outlet. Otherwise, the outlet pressure would eventually reach the
> input pressure. Only that scenario separates the normally single closed
> system into two "independent" systems.
>
> Ken H.
>
>
> On Sun, Jul 19, 2015 at 7:11 PM, A. wrote:
>
>> Neil Fonville wrote on Sun, 19 July 2015 17:58
>>> I have confirmed I have the rear port on the P30 connected to the rear
>> brakes. Tested pressure again. Got 800 rising to 900 when holding on
> the
>>> rear MC port. 500 steady at the rear output of the combination valve.
>>> Did not have parts to test at the input of the valve.
>>>
>>> Neil
>> You are within a couple shovels of paydirt. You got a 300 - 400 PSI drop
>> between the master cylinder and the output of the combination valve. I
> don't
>> know or care how that can even be possible. But you need to figure out
>> which component has the evil in it - the line or the valve. Disconnect
> the
>> line
>> between the master cylinder and the combination valve and verify there
>> isn't anything in it that would restrict flow. Then do the same between
> the
>> input and output of the combination valve. Might need to replace the
>> combination valve. Reassemble, bleed the rears one last time, and drive
> it.
>> --
>> '73 23' Sequoia For Sale
>> '73 23' CanyonLands For Sale
>> Upper Alabama
>> On "marriage equality": Once you define the direction that society is
>> headed as "forward", you can call it progress.
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> _______________________________________________
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| Re: [GMCnet] Disc brake project still in failure mode [message #282550 is a reply to message #282533] |
Sun, 19 July 2015 21:00 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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My bleeder pressurized both reservoirs with the same pressure. That won't
move the spool valve. Pumping the brake pedal sure will.
Jim Hupy
On Jul 19, 2015 5:34 PM, "A." wrote:
> Ken Henderson wrote on Sun, 19 July 2015 18:55
>> ...If the shuttle is indeed moving and blocking the rear outlet,...
>> Ken H.
> That would prevent one from bleeding the rear brakes. As soon as one opens
> a bleeder, the pressure would drop and the shuttle would block it off.
>
> When not bleeding, with the gauge attached, if it doesn't block it
> completely, the pressure would rise until it matches the pressure at the
> master
> cylinder.
>
> It either blocks it completely and you can't bleed the brakes, or not
> completely and the pressure will equalize.
>
> It still doesn't add up.
> --
> '73 23' Sequoia For Sale
> '73 23' CanyonLands For Sale
> Upper Alabama
> On "marriage equality": Once you define the direction that society is
> headed as "forward", you can call it progress.
>
> _______________________________________________
> GMCnet mailing list
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| Re: [GMCnet] Disc brake project still in failure mode [message #282559 is a reply to message #282550] |
Sun, 19 July 2015 21:47 |
A Hamilto
Messages: 4508
Registered: April 2011
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James Hupy wrote on Sun, 19 July 2015 21:00My bleeder pressurized both reservoirs with the same pressure. That won't
move the spool valve. Pumping the brake pedal sure will.
Jim Hupy As soon as one opens a bleeder, the pressure would drop on that side making it lower than the other side of the combination valve.
When not bleeding, with the gauge attached, if it doesn't block it completely, the pressure would rise until it matches the pressure at the master cylinder.
It either blocks it completely and you can't bleed the brakes, or not completely and the pressure will equalize.
What you are implying is the pressure bleeder pressurizes the lines to such a low point that the shuttle doesn't move enough to shut off the flow to the low pressure side.
But what it all comes down to, if what Mr. Henderson is proposing is reality, you can't pedal bleed a system with that PV4 combination valve. At least not one wheel at a time. You would have to open a bleeder on the front and rear at the same time, or the "shuttle", which is a switch piston in the manual, would shut off the flow to the wheel you are trying to bleed.
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| Re: [GMCnet] Disc brake project still in failure mode [message #282564 is a reply to message #282559] |
Sun, 19 July 2015 22:08 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Mark,
I brought one of Jim H's pressure bleeders to Bean Station where we bled the brakes on George Z's GMC after he installed the
reaction arm system he won at a GMCMI Convention.
We installed a clamp to hold the teat on the front of the combination valve in and then pressurized the bleeder to 10 psi.
We bled the calipers one at time in the following order:
1) Passenger side - Rear
2) Passenger side - Middle
3) Driver side - Rear
4) Driver side - Middle
5) Passenger side - Front
6) Driver side - Front
It took less than 20 minutes and we got ALL the air out of the system.
This scenario was repeated on JohnT's GMC earlier this year in Humble, Texas.
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: A.
What you are implying is the pressure bleeder pressurizes the lines to such a low point that the shuttle doesn't move enough to shut
off the flow to the low pressure side.
But what it all comes down to, if what Mr. Henderson is proposing is reality, you can't pedal bleed a system with that PV4
combination valve. At least not one wheel at a time. You would have to open a bleeder on the front and rear at the same time, or the
"shuttle", which is a switch piston in the manual, would shut off the flow to the wheel you are trying to bleed.
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Disc brake project still in failure mode [message #282565 is a reply to message #282564] |
Sun, 19 July 2015 22:51 |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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Senior Member |
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USAussie wrote on Sun, 19 July 2015 22:08Mark,
I brought one of Jim H's pressure bleeders to Bean Station where we bled the brakes on George Z's GMC after he installed the
reaction arm system he won at a GMCMI Convention.
We installed a clamp to hold the teat on the front of the combination valve in and then pressurized the bleeder to 10 psi.
We bled the calipers one at time in the following order:
1) Passenger side - Rear
2) Passenger side - Middle
3) Driver side - Rear
4) Driver side - Middle
5) Passenger side - Front
6) Driver side - Front
It took less than 20 minutes and we got ALL the air out of the system.
This scenario was repeated on JohnT's GMC earlier this year in Humble, Texas.
Regards,
Rob M.
The Pedantic Mechanic The pressure bleeder is the way to go. But if I understand what Mr. Henderson wrote, the switch piston can completely shut off the flow of brake fluid to one side if the differential is great enough. Yes that is good to stem the loss of fluid if there is a leak, but it also means it can prevent you from pedal bleeding the brakes. I never encountered that before.
But if I had to conjecture how that could account for the pressures Neil has observed at assorted points in the line, that scenario would start with the premise that there is an air bubble in the rear. As the pedal is pressed, the bubble compresses and pressure builds to 500 PSI. BUT, at that point, the front pressure is higher enough that the "shuttle" has moved far enough to shut the rear line off from the rest of the system, and the pressure between the master cylinder shuttle continues upward to 800 PSI. So holding the pedal down, you got 800 PSI from the master cylinder to the combination valve and 500 PSI from the combination valve to the rear calipers. And it stays that way until you disturb the system.
Opening a rear caliper bleeder would leak it down, then the fluid would stop flowing, even while there was still 800 PSI at the master cylinder, and the pedal would not move at all.
Like I said, I have never seen that.
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| Re: [GMCnet] Disc brake project still in failure mode [message #283122 is a reply to message #281955] |
Sat, 25 July 2015 08:55 |
Tom Whitton
Messages: 235
Registered: February 2004
Location: Paducah, KY
Karma: 0
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Senior Member |
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Neil,
Congratulations on the improvement in your brakes. 800 and 700 PSI really aren't bad readings with a vacuum booster system. You could still have a small amount of air in the rear lines. However, to get significantly more pressure you would need to go to the Hydroboost system, which most say provides car-like braking.
Read here: http://appliedgmc.com/images/hydro.pdf
Tom Whitton
26 ft updated GMC
Paducah, KY
Date: Sat, 25 Jul 2015 12:07:36 +0000
From: Neil Fonville
To: "gmclist@list.gmcnet.org"
Subject: Re: [GMCnet] Disc brake project still in failure mode
Message-ID:
Content-Type: text/plain; charset="us-ascii"
Update: So I installed the new combination valve using the screw in lock
tool while bleeding. The screw in lock tool keeps the internal pin from
moving and replaces the brake light connection during installation. Used
the Hupy bleeder and bleed the system by farther to closest to MC. Very
much improved. The coach is drivable and braking action is pretty good
but not great. I will drive it some this weekend to see if it improves
with settling in of the brake pads.
I have a few questions:
I get 800 psi at the MC but only 700 at the RR caliper. Does that mean I
still have a bit of air or tiny leak?
Since the MC is only pushing 800 psi. Would that be my max throughout
the system? Can anything be done to improve that?
The pedal has to be much farther in the stroke for the braking action to
occur. Did I not get the pin length long enough or incorrect?
Thanks for all the input during this.
Neil
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