Combination Valve - Proportioning also?

[billvv]

Wondering if any one knows whether the brake Combination Valve (steel factory-installed version) included a proportioning valve. It is not mentioned
in the WSM. I know it has the metering valve, bleed button, shuttle, and alarm switch; just can't tell whether the 'brass seat' in the photo archive
(link below) is a proportioning valve and if not, whether it may have been removed at some point.

http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[jim kanomata1]

Bill,
The original and the new brass has that feature built in.
Most of the steel ones have been contaminated with moisture and are not
working too well.
The fluid need to be flushed out on the coach frequently if you ride the
brake as when the fluid gets hot the moisture becomes steam and you
cannot get much pressure transfered through steam.

On Thu, Dec 24, 2020 at 3:11 PM Bill Van Vlack via Gmclist <

> Wondering if any one knows whether the brake Combination Valve (steel
> factory-installed version) included a proportioning valve. It is not
> mentioned
> in the WSM. I know it has the metering valve, bleed button, shuttle, and
> alarm switch; just can't tell whether the 'brass seat' in the photo archive
> (link below) is a proportioning valve and if not, whether it may have been
> removed at some point.
>
>
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502

 

[Jim Hupy]

Bill, think of it more as a "delay" valve and a isolation device. The delay
function prevents application of the disc pads on the front, until the rear
brake shoes have traveled into intimate contact with the rear drums. Then
pressure is applied equally from the tandem master cylinder to both ends of
the coach. The isolation function is to keep the fluid from the rear
chamber separate from the fluid from the front chamber. That switch and
toggle assy tells you via the dash warning lamp if you have a loss of
pressure in one end or the other. The master cylinder pistons are of equal
diameter, so the pressure applied is equal to both ends of the coach. No
restriction exists in the distribution valve. Which it is, in technical
terms, not a pressure limiting device. Many have argued back and forth over
this. When we do an all disc system, we often remove the "delay" portion so
all the brakes apply at the same time. Personally, I have done that
conversion without altering the valve, and can't tell much difference
either way. Some guys think there is some improvement by disabling, some do
not. You still have a 10,000 pound vehicle to stop amongst 4 wheel cars
that are tons lighter and are equipped with ABS and computer controlled
systems. If one of those things locks 'em up in front of you, you are going
to have a new hood ornament. Basic physics 101.
Jim Hupy
Salem, Oregon

On Thu, Dec 24, 2020, 3:11 PM Bill Van Vlack via Gmclist <

> Wondering if any one knows whether the brake Combination Valve (steel
> factory-installed version) included a proportioning valve. It is not
> mentioned
> in the WSM. I know it has the metering valve, bleed button, shuttle, and
> alarm switch; just can't tell whether the 'brass seat' in the photo archive
> (link below) is a proportioning valve and if not, whether it may have been
> removed at some point.
>
>
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[billvv]

Thank you Jim. So there is no proportioning (or bias) done in the factory valve. Any thoughts about the replacement brass valves vis-a-vis
having/not having a proportioning valve?

When the metering (delay) section is removed, I believe the bleed button/shaft is removed as well; does the system bleed through when one side is not
holding pressure (and when using your excellent pressure bleeder, of course) or do you need to insert a 'stopper' where the light switch goes to pin
the isolation shuttle in place?
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[Jim Hupy]

Nope. It takes more than the pressure of my bleeder tank to move that spool
valve against the spring. Depending upon how crudded up the valve is due to
rust, etc. Even when all is OK inside, it still takes more pressure than 10
psi. How much more pressure? Depends on the springs.
Jim Hupy
Salem, Oregon

On Thu, Dec 24, 2020, 4:47 PM Bill Van Vlack via Gmclist <

> Thank you Jim. So there is no proportioning (or bias) done in the factory
> valve. Any thoughts about the replacement brass valves vis-a-vis
> having/not having a proportioning valve?
>
> When the metering (delay) section is removed, I believe the bleed
> button/shaft is removed as well; does the system bleed through when one
> side is not
> holding pressure (and when using your excellent pressure bleeder, of
> course) or do you need to insert a 'stopper' where the light switch goes to
> pin
> the isolation shuttle in place?
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[billvv]

So that button (if installed) is only needed when brake pedal bleeding, if I understand correctly.
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[Jim Hupy]

It allows the front brakes to be bled, when pushed in. Often frozen up. I
use a few light taps with a machinists hammer, and copious amounts of my
favorite penetrant to free it up. There is a tool pictured in the manual
that holds the button in while bleeding. Easily home built.
Jim Hupy
Salem, Oregon

On Thu, Dec 24, 2020, 6:12 PM Bill Van Vlack via Gmclist <

> So that button (if installed) is only needed when brake pedal bleeding, if
> I understand correctly.
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Matt Colie]

We easterners have the advantage that Dave Lenzi is usually at the Internationals with his demonstration rig. He can show, but it has to be fast,
that the "hold off' in that combination valve is about 200psi and once that happens, both ends are the same.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

 

[billvv]

Matt,
Meaning that there is no proportioning section to reduce pressure to the rear brakes in the steel combination valve, if I understand your comment
correctly.

Bill
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[Matt Colie]

> Matt,
> Meaning that there is no proportioning section to reduce pressure to the rear brakes in the steel combination valve, if I understand your comment
> correctly.
>
> Bill

Bill,

That is true and I have seen it so in Dave's test rig. It goes by real fast. If there, you can watch both gauges and the front won't move until set
pressure (~200psi). Then Baamm, the same pressure is at the front.

Matt

--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

 

[billvv]

Matt,
Were any of the brass combination valves tested?

On another tack..... I understand that some combo valves have a 'spike killer' function that limits pressure to the rear brakes on panic stops only. I
am wondering if the testing simulated a panic stop.

Bill
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[john r. lebetski]

Bill I reread Jim Ks response. Sounds to me like he is saying the steel and brass both function the same The brass fairs better with saturated
corrosive fluid over time. .
Is that correct Jim K?
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[Matt Colie]

> Matt,
> Were any of the brass combination valves tested?
>
> On another tack..... I understand that some combo valves have a 'spike killer' function that limits pressure to the rear brakes on panic stops
> only. I am wondering if the testing simulated a panic stop.
>
> Bill

Bill,

The combination valve that I had apart had two internal features.
One I expected was the floating piston that will push the switch if there is a difference in pressure between the front and rear circuits.
The other one I did not expect to be as involved as it was is the front part. That has a set of parts that are a lot like a "blow down" set of a
boiler safety valve. When a set pressure is reached,the valve partly opens and exposes more area to the pressure so it immediately opens more. There
is also a balance chamber in this so the pressure has to get very close to Zero before the system will reset.

I didn't take pictures. If I can find the box it is in, I will take pictures and publish them.

I do not recall any thing other than passages for the rear brake circuit. When I find the parts, I will look again.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

 

[blur911sc]

In industrial hydraulics language, it would be called a sequence valve. Plus of course the switch to tell if either hydraulic circuit has air or a
leak.

auto=format&fit=max&w=1440

 

[Ken H.]

Bill,

Looking back at my combination valve album, I can see where your curiosity
about the "brass valve seat" could arise. First off, I'm sure that
everything you see in the album is original. BUT, because the "...seat"
appears to be much longer than necessary, I too now wonder if, in another
application, that space may have been used to incorporate a proportioning
valve for the rear brakes. Having never had such a device apart, I don't
know the details, but I suspect that a spring and other components could be
fitted into a space similar to the excess occupied by that overly-long
seat. But, rest assured that the GMC NEVER had a proportioning valve.
Besides all the documentation to that effect, think about the reason for
such a valve: to prevent the rear tires from losing traction before the
front ones. With the GMC's FOUR rear tires, there was never much chance of
them locking up before the two front ones, so why bother with a
proportioning valve?

JMHO,

Ken H.

On Thu, Dec 24, 2020 at 6:11 PM Bill Van Vlack via Gmclist <

> Wondering if any one knows whether the brake Combination Valve (steel
> factory-installed version) included a proportioning valve. It is not
> mentioned
> in the WSM. I know it has the metering valve, bleed button, shuttle, and
> alarm switch; just can't tell whether the 'brass seat' in the photo archive
> (link below) is a proportioning valve and if not, whether it may have been
> removed at some point.
>
>
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[billvv]

I think my question has been answered to my satisfaction... The steel combination valve does not have a proportioning function. It will pass whatever
pressure is delivered to the input port directly to the output port.

What convinced me was Matt's input, other online reports of pressure testing, and a short video that Bob Stone made showing full pressure of his
Hydroboost being delivered to the rear wheels.

Presumably - contrary to how combo valves were set up in other GM vehicles of the era - GM weighed all the items affecting brake performance and
lockup (slave cylinder diameter, drum brake design, front/rear weight distribution, suspension design, others?) for the GMC Motorhome and decided that
it was not necessary (or desired?) to limit MC pressures to the rear brake drums.

For the metering section, it's clear to me that for rear disk modifications, the metering section of the steel combination valve is not needed or
desired. Front brakes do not need to be delayed to allow drum springs to 'catch up'. I suppose that's true for the brass combo/proportioning valves
being sold as replacements for the steel valve as well.

I do still have a question about the remaining section of the steel combo valve - the spool section that lights the brake warning light. In addition
to activating the light, does that section also block off MC pressure to the section (front or rear) that has lost brake pressure?

--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[Ken H.]

Bill,

It's pretty obvious from bit.ly/38BItrj that when the switch-actuating
shuttle moves, it can only close the switch -- NOT any hydraulic port. So,
if front or rear brakes fail open, that master cylinder reservoir will be
emptied. Which is a good reason to NOT do what I've seen people suggest:
Drilling through the wall between the two reservoir sections to allow
sharing.

Ken H.

On Sun, Dec 27, 2020 at 2:56 PM Bill Van Vlack via Gmclist <

> I think my question has been answered to my satisfaction... The steel
> combination valve does not have a proportioning function. It will pass
> whatever
> pressure is delivered to the input port directly to the output port.
>
> What convinced me was Matt's input, other online reports of pressure
> testing, and a short video that Bob Stone made showing full pressure of his
> Hydroboost being delivered to the rear wheels.
>
> Presumably - contrary to how combo valves were set up in other GM vehicles
> of the era - GM weighed all the items affecting brake performance and
> lockup (slave cylinder diameter, drum brake design, front/rear weight
> distribution, suspension design, others?) for the GMC Motorhome and decided
> that
> it was not necessary (or desired?) to limit MC pressures to the rear brake
> drums.
>
> For the metering section, it's clear to me that for rear disk
> modifications, the metering section of the steel combination valve is not
> needed or
> desired. Front brakes do not need to be delayed to allow drum springs to
> 'catch up'. I suppose that's true for the brass combo/proportioning valves
> being sold as replacements for the steel valve as well.
>
> I do still have a question about the remaining section of the steel combo
> valve - the spool section that lights the brake warning light. In addition
> to activating the light, does that section also block off MC pressure to
> the section (front or rear) that has lost brake pressure?
>
>
> --
> Bill Van Vlack
> '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
> Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
> mid
> November 2015.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[billvv]

Thanks, Ken;
That seems to align with other info I've seen, so to summarize...

The steel OEM combo valve has a metering section (to hold back the front brake action and let the rear drum springs compress) and an alarm section (to
alarm when the rear brake line pressure is lower than the front brake line pressure or vice versa).

The alarm section does not isolate the low pressure side when activated.

Jim Hupy and others remove the metering section when rear disks are installed.

We don't have any reports stating what features are included the several varieties of the brass combination (or proportion) valves although it's
likely, given the alarm terminal, that it has an alarm section. Some cutaways show what may be a metering section but do not show what, if anything,
is in the rear brake section of the valve where a proportioning section might live.

--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[john r. lebetski]

I spoke to Bob Stone, as he is not on the web, and he is looking into all this more deeply, time and weather permitting. Hoping to get old, new and
PV2 dissected and at some point pressure readings.

--
John Lebetski
Woodstock, IL
77 Eleganza II