| [GMCnet] Update on my Faded Brakes [message #69093] |
Thu, 31 December 2009 11:39  |
fbhtxak
Messages: 191
Registered: April 2006
Karma: 0
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Senior Member |
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Dan,
'Just clicked on this version of the GMCNet Digest...'Sorry
to find that this problem continues to frustrate you and
Terri.
I agree with Ken H. on his counsel to determine which system
is not getting pressure. Until you do this, anything else
you do will, in my experience, be out of sequence in
diagnosing the problem. If there is no visible evidence of a
leak, along the routing of the brake lines or at the wheel
backing plates, you will need to remove wheels (and drums at
the rear) until you find the problem(s).
The combination valve piston may have shifted but its switch
may not have activated the brake warning light on the gauge
panel. It is activated so infrequently that either the
switch does not complete the circuit or the bulb is
defective. In my case, the bulb did NOT light when I
completed the circuit to ground during my test of the
switch. "Rocking" the bulb in its base, however turned the
light on. It was likely the first time the light was ever
activated (I have had the vehicle since almost new - 24k mi.
when I got it).
If the valve piston has shifted due to a failure in either
the front or rear system, it will not return to center until
the failure is fixed. It will "center" under pressure when
both front and rear systems are working properly. You will,
however, have to center the valve manually to bleed the
repaired system. My leak was so slow (trailing driver side
rear wheel) that the valve did not shift. Bleeding the
replacement brake cylinder then was not a problem.
My experience with master cylinder failure is that, under
pressure, the pedal will slowly sink to the bottom of its
travel. That is often due to internal recirculation as a
result of seal failure(s). When this happens, rapid pumping
of the pedal can often compensate for the recirculation
enough to safely stop the vehicle. From your reports, it
doesn't appear that you have a master cylinder failure - or
at least one due to recirculation.
Here is an excerpt from the X7525 Service manual on the
combination valve:
"COMBINATION VALVE
A combination is incorporated into the brake system.
It performs three functions; a balance function,
a metering function, and a warning switch .
METERING VALVE FUNCTION
The metering section of the combination valve
operates to "hold off' hydraulic flow (pressure) until
about 130 psi has been built up in the system before
applying the front brakes. The pressure then blends
to full line pressure at approx. 400-600 psi line pressure.
There is no flow restriction when the brakes are
released."
Fred Hudspeth
'78 Royale - Tyler, TX
'82 Airstream Excella 28' Motorhome - Anchorage, Alaska
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| Re: [GMCnet] Update On My Faded Brakes [message #69240 is a reply to message #69093] |
Fri, 01 January 2010 17:31  |
James Moore
Messages: 71
Registered: January 2004
Karma: 0
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Member |
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Dan,
I haven't been following this thread in detail so please excuse me if I
repeat information sent to you by others.
As the rear brakes wear, the brake pedal will move closer to the floor. As
this happens, the pressure in the rear brake system will decline. If the
max pressure in the rear lines declines to less than 130 psi, the metering
valve will not allow pressure to the front calipers and the rear brakes are
doing all the stopping. This sounds like what is happening to you and is
the reason Gene had to adjust his rear brakes to maintain good performance
before he switched to rear discs.
The rear brakes only self adjust when the GMC is backing up and the brakes
are applied. Since most GMCs are not backed very often, the rear brakes
don't self adjust. Before I would start spending money replacing parts, I
would back the GMC and apply the brakes 10 or 15 times and see if that
improved things. However if the adjusting screws are frozen due to lack of
use, the brakes may not adjust. I try to back my GMC every couple of days
and apply the brakes a few times when I am on a trip. I have to do this
when the towed isn't attached.
Here is a link to a site with lots of good information on trouble shooting
brakes. http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm
Good luck.
Jim Moore
75 PB
Battle Ground, WA
> [Original Message]
> From: Dan Gregg <gregg_dan@hotmail.com>
> To: <gmclist@temp.gmcnet.org>
> Date: 12/31/2009 1:36:17 PM
> Subject: Re: [GMCnet] Update On My Faded Brakes
>
>
>
> Arch's link is gone. I have a 74 built in 73. There is a bracket with two
bolts holding the master cyl. up, in addition to the two bolts to the
booster. Is this factory? If so, just pull those two also? I am assuming
they are metal screws.
> Dan
> --
> Dan & Teri Gregg
>
> danandteri.blogspot.com
>
>
>
> ///Halon Automatic Fire Extinguishers
> _______________________________________________
> GMCnet mailing list
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| Re: [GMCnet] Update On My Faded Brakes [message #69253 is a reply to message #69240] |
Fri, 01 January 2010 19:51 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""The rear brakes only self adjust when the GMC is backing up and the brakes
are applied. Since most GMCs are not backed very often, the rear brakes
don't self adjust. Before I would start spending money replacing parts, I
would back the GMC and apply the brakes 10 or 15 times and see if that
improved things. However if the adjusting screws are frozen due to lack of
use, the brakes may not adjust. I try to back my GMC every couple of days
and apply the brakes a few times when I am on a trip. I have to do this
when the towed isn't attached. ""
Jim--that's my thinking as well. I have to adjust my rear brakes quite often out here (in the mountains of AZ) even though I try to give them some good back up exercises. People look at me like I'm nuts when I do that in a semi empty parking lot. However, even though my adjusters appear to be in excellent shape, I have much better luck doing them manually. Tomorrow we hope to see if this may be happening with Dan's coach--if not we'll go to plan B and then plan C whatever that may be:)
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Update On My Faded Brakes [message #69281 is a reply to message #69253] |
Sat, 02 January 2010 01:16 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Thanks for going to help Dan. If he was a little closer I would have offered to help him also.
Hebron, IN to Duncan, AZ = 1692 miles
It is still around 200 miles for you. At least it will not be 0 to +15 degrees when you get there.
Thanks
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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