| [GMCnet] Update on my Faded Brakes [message #69093] |
Thu, 31 December 2009 11:39  |
fbhtxak
Messages: 191
Registered: April 2006
Karma:
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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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