Correct (and hopefully NON-leaking) brass combination valve?

L

larry engelbrecht

New member
Nov 20, 2006
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I've been reading about brass combination valves that leak, supposedly the Chinese-made PV2 (I have the stock disc/drum setup). Seems the GENUINE GM
1257177 is nearly impossible to find, and many of the hits when I enter "AC Delco 172-1350" actually say words to the effect that it's made identical
to, "AC Delco TYPE" (caps mine), which doesn't convincingly sound like a genuine AC Delco part. I've also read on this board that one of the fittings
strips easily, so I'm understandably hesitant to figure out exactly which one to purchase.

Are all the PV2s leakers? I did find one source claiming it's part is made by AC Delco, so that seems to be the safest bet at this point, but I'd like
to hear what the community says.

Any advise? Thanks in advance.
--
Larry Engelbrecht

San Diego, CA

'73 26' ex-Glacier

TZE063V100319 03/07/73
 
The description indicates that it is for '4-wheel disk brakes'.
--
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.
 
> The description indicates that it is for '4-wheel disk brakes'.

This part?
Brand-new solid brass discontinued new-old-stock genuine GM Brake Combination Valve for various GM front disc/rear drum brake applications.

GM # 1257177 ACDelco # 172-1350

We are aware that this brake valve fits 1973-1978 GMC Motorhomes Vin TZE, and also 1974 Super Duty (SD) 455 Trans Am.
--
JD Lisenby- USAF Ret
1978 Royale-455
MacDash, Manny Tranny, FI-tech, 3.70 etc etc

Navarre, FL
 
You are correct! I scrolled down too quickly to the $37.00 disk/disk version.
--
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.
 
Highway Stars is my friend’s company. Bob Stone was having multiple issues with the Chinese PV2s. So as an ACD dealer I had Highway Stars source
them in system and buy them up for him. We put the remaining units on the Highway Stars web site. GM /ACD new in box. This also crosses to the 74
SD455 Trans Am which was Disc /Drum before they went all disc. Bob gave us the part numbers to source these. And yes the PV2 MBM parts are also sold
on his page for the Regal crowd so be sure NOT to click on any of those.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Does anyone know if this would be the same combination valve for a coach with a rear disk conversion?
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX
 
It’s listed disc drum. The disc disc would not have the hold off feature the same as disc drum. For those with disc mid and drum rear? Who’s to
know because GM did not equip them that way.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Thanks to all for the advice. I did order one from Highway Stars. Not cheap, but I hate redoing things so that peace of mind alone makes it worth the
price.
--
Larry Engelbrecht

San Diego, CA

'73 26' ex-Glacier

TZE063V100319 03/07/73
 
My belief in 2007 was that the combination valve had only three functions:
1. Hold off activation of the front disc brakes until 135 psi of line
pressure activated the rear drums. 2. Activate the differential pressure
switch if one system lost pressure; and, 3. Provide a convenient place for
all the plumbing to come together. My subsequent experience has seemed to
verify that, so I don't know what more a disc/disc valve does -- can
someone enlighten me? If there's not another function, perhaps more folks
will want to merely "rebuild" and modify their OEM disc/drum valve IAW this
example:
http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html

Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL,
Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
www.gmcwipersetc.com

On Wed, Sep 16, 2020 at 6:26 PM Larry Engelbrecht via Gmclist <

> Thanks to all for the advice. I did order one from Highway Stars. Not
> cheap, but I hate redoing things so that peace of mind alone makes it worth
> the
> price.
> --
> Larry Engelbrecht
>
> San Diego, CA
>
> '73 26' ex-Glacier
>
> TZE063V100319 03/07/73
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Hi, Ken.

IIRC, drums (or the drum side of a disc/drum system) combination valves
also have a residual (?) valve, retaining 2-10 psi in the line to prevent
excessive piston, and consequently, shoe retraction when brakes are
released. This also may complement the metering function you first
described.

Prior to dual systems beginning in '67 (???), single master cylinder system
drum/drum systems probably had nothing more than that metering valve built
into the single master cylinder.

Thank you.

Larry Engelbrecht

On Wed, Sep 16, 2020 at 3:52 PM Ken Henderson
wrote:

> My belief in 2007 was that the combination valve had only three
> functions: 1. Hold off activation of the front disc brakes until 135 psi
> of line pressure activated the rear drums. 2. Activate the differential
> pressure switch if one system lost pressure; and, 3. Provide a convenient
> place for all the plumbing to come together. My subsequent experience has
> seemed to verify that, so I don't know what more a disc/disc valve does --
> can someone enlighten me? If there's not another function, perhaps more
> folks will want to merely "rebuild" and modify their OEM disc/drum valve
> IAW this example:
>
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
>
> Ken H.
> Americus, GA
> '76 X-Birchaven w/Cad500/Howell EFI & EBL,
> Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
> www.gmcwipersetc.com
>
>
> On Wed, Sep 16, 2020 at 6:26 PM Larry Engelbrecht via Gmclist <

>
>> Thanks to all for the advice. I did order one from Highway Stars. Not
>> cheap, but I hate redoing things so that peace of mind alone makes it worth
>> the
>> price.
>> --
>> Larry Engelbrecht
>>
>> San Diego, CA
>>
>> '73 26' ex-Glacier
>>
>> TZE063V100319 03/07/73
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
 
Ken on #2 besides toggling the valve if one side fails it also grounds the brake light to come on. So 2 is really 2 things if that’s what you meant.

Also to comment on the price from the other post, he’s selling below. MSRP.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Well, I did get to learn at least one thing today! I didn't even think
about the 2 psi "hold on" pressure for the discs -- thinking back, that's
probably a second purpose for the little relief valve actuated by the pin
on the front of the combination valve. But the only "Brake Light" I can
find actuated by the differential pressure switch is that connected to pin
E on the instrument cluster -- not those on the rear of the coach, which is
what I first thought John was telling us about. But John's right about
that being a second function because the REALLY important one is shutting
off the non-functional brake system to limit fluid loss -- IIRC.

Ken H.

On Wed, Sep 16, 2020 at 7:12 PM John R. Lebetski via Gmclist <

> Ken on #2 besides toggling the valve if one side fails it also grounds the
> brake light to come on. So 2 is really 2 things if that’s what you meant.
>
> Also to comment on the price from the other post, he’s selling below.
> MSRP.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> My belief in 2007 was that the combination valve had only three functions:
> 1. Hold off activation of the front disc brakes until 135 psi of line
> pressure activated the rear drums. 2. Activate the differential pressure
> switch if one system lost pressure; and, 3. Provide a convenient place for
> all the plumbing to come together. My subsequent experience has seemed to
> verify that, so I don't know what more a disc/disc valve does -- can
> someone enlighten me? If there's not another function, perhaps more folks
> will want to merely "rebuild" and modify their OEM disc/drum valve IAW this
> example:
> http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html
>
> Ken H.
> Americus, GA
> '76 X-Birchaven w/Cad500/Howell EFI & EBL,
> Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
> www.gmcwipersetc.com

It was always my understanding those valves also perform a brake bias function as well.... That's what I was most concerned about. Unless the disc
conversion was designed with calipers that mimicked the hydraulic requirements of the drums, the brake bias would be off unless a change to the
proportioning valve was also made....

I inherited my disc conversion from the PO, so not sure if it was designed with this in mind, or if a different combination valve was used when the
disc conversion kit was installed...

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX
 
> I inherited my disc conversion from the PO, so not sure if it was designed with this in mind, or if a different combination valve was used when
> the disc conversion kit was installed...

And I guess the 3rd option is brake bias was not even considered in designing the conversion, in which case I am just not getting full brake
performance out of my system....

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX
 
I wish Bob Stone was on line here. I have to ask him but most biasing is AFAIK determined by the master. He has been measuring various MCs and
coming up with interesting data. One thing is if the biasing is off or inverted, it trips the combination valve, as it is just doing it’s job.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Jim Kanomata appliergmc.com http://appliergmc.com/ carries both the disc/drum and the disc/disc combination valves.

Emery Stora

>
> Does anyone know if this would be the same combination valve for a coach with a rear disk conversion?
> --
> Mark S. '73 Painted Desert,
> Manny 1 Ton Front End,
> Howell Injection,
> Leigh Harrison 4bag and Rear Brakes,
> Fort Worth, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
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