| Re: [GMCnet] update on master cylinder replace [message #198353] |
Wed, 13 February 2013 16:02  |
KB
Messages: 1262
Registered: September 2009
Karma: 0
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Senior Member |
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I think I said it correctly. Drum brake wheel cylinders may (and
that's the question) need different valving in the master cylinder than
disc brake calipers do. Disc brake calipers have o-rings. Drum brake
wheel cylinders have cup seals. So a master cylinder designed only for use
with disc brakes may not be a good thing to use with drum brakes.
Or at least that's the theory/question.
The point I was trying to resolve is what master cylinder to use
when you have drum brakes but expect to upgrade to disc brakes.
thanks
Karen
1973 23'
1975 26'
KenH wrote:
> I'm confused by that statement/question: You speak of disc master
> cylinders and wheel "cylinders" rather than "calipers". Calipers do not
> need residual pressure valves unless the master cylinder is located below
> the calipers, as on some hot rods. And the purpose is not to retract the
> pads but to prevent them from retracting too much when the fluid runs back
> to the master cylinder. Normally, caliper pistons are retracted by the
> hysteresis of their rubber seals, by disc run-out, and by play in the wheel
> bearings.\
> On Mon, Feb 11, 2013 at 3:17 PM, KB wrote:
>
>> > As long as I'm bothering the experts with silly questions... :-)
>> >
>> > If I understand it, one should not use master cylinders designed for an
>> > all-disc
>> > system on a vehicle that has drum brakes, because some (most? all?) wheel
>> > cylinders need residual pressure to retract fully and keep the cup seals
>> > closed.
>> > Most disc-disc master cylinders don't include the necessary valve(s).
>> > This could
>> > result in dragging brakes and/or air in the wheel cylinders.
>> >
>> > Any truth to this??
>> >
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] update on master cylinder replace [message #198361 is a reply to message #198353] |
Wed, 13 February 2013 17:28  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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G'day,
I just read Maintenance Manual (MM) X-7525, Section 5 Brakes, Pages 5-1 through 5-13 which covers General Information and Brake
System Trouble Diagnosis. I also skimmed pages 5-14 through 5-46 and I could NOT find any reference to a residual valve anywhere. I
then moved on to MM X-7625 and MM X-7725 there was no reference in either one of those manuals.
Returning to MM X-7525 Section 5, Page 5-27 I found Figure 32 - Master Cylinder (Exploded View). There is no residual valve in that
figure. There is, however, two "inserts" in the ports that feed the combination valve. It is possible that there are residual valves
in those inserts. However, considering the level of detail shown in the rest of the figure I don't believe there is.
I then checked Parts Book 78Z, Section 4 Brakes, Page 4-10, Figure 4.010 - Cylinder - Master, and it had the same level of detail.
It shows Key 5 - INSERT - tube fitting.
It seems to me that if either one of Key 5 were residual valves the parts book would call them residual valves NOT tube fitting.
IIRC when looking at a GMC Master Cylinder the insert is made of brass.
I did a bit more research into the way the brakes on a GMC work and noted a couple of things that could / would negate the necessity
for a residual valve.
1) Here's a cross section of the Combination valve that will be helpful to reference when reading the paragraph below.
http://www.gmcmhphotos.com/photos/member-galleries/p37696-combination-valve-cros.html
Ref: MM X-7525, Section 5 Brakes, Page 5-3 COMBINATION VALVE - 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 a full line pressure at approximately 400-600 psi line
pressure. There is no flow restriction when the brakes are released.
The metering valve is the spring and poppet on left side of the brass combination valve.
Below is a photograph that KenH took showing the internal parts of the OEM Combination valve. From what I can see I don’t believe
there is a residual valve in it. There could be one in the part Ken called the Brass Seat (part with screw extractor in it).
http://www.gmcmhphotos.com/photos/deciphering-the-combination-valve/p5329.html
or in the port on the left of this photo:
http://www.gmcmhphotos.com/photos/deciphering-the-combination-valve/p5323.html
But to me it looks like a clear passageway.
2) The rear brake shoes automatically adjust when the brakes are applied when backing up which leads me to assume they are kept
close enough to the drums to provide braking without having to depress the brake pedal very far.
Well' that's my $0.02's on the subject.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
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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] update on master cylinder replace [message #198478 is a reply to message #198467] |
Thu, 14 February 2013 14:44 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Bill,
Thank you for posting a picture of the Jeep master cylinder and please don’t take the following negatively.
I am aware that many master cylinders have residual valves in the master cylinder ports; however, I don’t see how that is relevant
to a GMC.
Does anyone have a bad OEM master cylinder that they can remove the brass inserts from and see what's under them to answer this
question?
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Bill Wevers
Sent: Friday, February 15, 2013 6:48 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] update on master cylinder replace
Hello Rob,
I haven't looked at the GMC master cylinder output port.
But here's a photo of a Jeep master cylinder with brass inserts
pressed into the ports. They are the residual check valves that
must be removed when converting to disk brakes.
Regards,
Bill
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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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