Problem with brass body proportioning valve

Matt Colie

Matt Colie

Well-known member
Aug 15, 2008
11,101
807
113
South East Michigan near DTW
If you have the replacement brake differential indicator and hold-off valve (it is not supposed to be a proportioning valve) and you have not read the
wonderful and very complete article post by Dave Lenzi, you should do that today and plan to complete the suggested mod as soon as it is possible (not
just practical).

Executive Summary:
The Brass replacement parts are not correct for TZE at all. While they do the "hold-off", they also reduce the pressure delivered to the rear brakes.
This is not something we need or want. The modification required to correct this looks simple enough. It can probably be completed with the valve
in place, but I won't know that for a fact for a few days.

I will read the article again (probably several times) before I do the mod and mark the valve as done.

This annoys me because somehow I did not see the two issue of VR come into the house and so never read them. I am hoping it is just a silly mistake.

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
 
First, we should call it a combination valve as proper terminology. The currently off shore produced MBM Brake PV2 disc drum combination valve that
Cinnabar and others install (with proportioning!) has been found to have a very high multiple mode fail rate. They have visibly smaller fluid ports as
well. The NOS GM brass valve number was sourced by Bob Stone and has had zero known defects over more that 10 years. The rear section rubber can
easily be removed by end user and the “proportioning” function defeated, while retaining the hold off, safety shuttle valve and light switching
(grounding) functions. I would talk to Bob or if you go on line to CPP (classicperform.com) there is a diagram with notes on removing the ‘rubber
piston’. There is a “drought” of good options out there as GM Restoration does not license brake related parts, so no incentive to get them made
by quality shops as reproductions. Also remember GM phased out steel valves (like our original) due to corrosion. Some 80’s GM cars were still
delivered with steel combination valves, but by say 1990 all service parts were brass. Most GM valves are now depleted.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
> First, we should call it a combination valve as proper terminology. The currently off shore produced MBM Brake PV2 disc drum combination valve
> that Cinnabar and others install (with proportioning!) has been found to have a very high multiple mode fail rate. They have visibly smaller fluid
> ports as well. The NOS GM brass valve number was sourced by Bob Stone and has had zero known defects over more that 10 years. The rear section
> rubber can easily be removed by end user and the “proportioning” function defeated, while retaining the hold off, safety shuttle valve and light
> switching (grounding) functions. I would talk to Bob or if you go on line to CPP (classicperform.com) there is a diagram with notes on removing the
> ‘rubber piston’. There is a “drought” of good options out there as GM Restoration does not license brake related parts, so no incentive to
> get them made by quality shops as reproductions. Also remember GM phased out steel valves (like our original) due to corrosion. Some 80’s GM cars
> were still delivered with steel combination valves, but by say 1990 all service parts were brass. Most GM valves are now depleted.

John,

I went to the link and I found two others that were about adding proportioning valves, but did not find anything that related to out issue. It could
well be there and I just did not open the right article. Can you hint please?

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
 
Yes. That’s it. Link is good. To big to post as a screen shot. Thanks.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Still fighting w/ my brakes and, once again, perfect timing for this post. Literally putting in my new valve tomorrow. Did the mod. Super easy.
Fingers crossed.

Shawn
--
Shawn Harris
North Vancouver,
Canada
1977 Palm Beach 403
 
So are you having to bleed the brakes after this mod?
--
M Beam
75’ Avion
TBI EBL , 3.70 LSD and other stuff
Zuki Sidekick,
Dozier Al
 
Well yet again Dave Lenxi hits a home run with his issue 154 (article) To
further support his findings if you look up the distribution valve in the
parts book it calls out ; "VALVE ASSY-metering balance and distribution
switch (SOA)(repl1236005)" It DOES NOT describe any rear brake pressure
limiting feature. DO NOT use the word PROPORTIONING when describing the
GMC MH distribution or combination valve. It is not supposed to limit
pressure in any way to the rear brake channel. The real problem that
should concern us all is, the available replacements for the GMC METERING
BALANCE AND DISTRIBUTION SWITCH valve assy are generically called PV2 or
PV4. These all have pressure limiting features to the rear wheels.
Without a direct replacement of the Cast iron Metering balance /
distribution valve we can follow the alteration/modification instructions
shown in:
https://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf

Again Dave uncovers a well hidden piece of knowledge, a tool that is
supposed to be employed when bleeding the brakes. You are supposed to
remove the electrical switch from the valve body and insert a tool in its
place. The tool then restricts the shuttle effect that actuates the switch
for the dash brake light. Holding that "shuttle" allows for more effective
rear brake Bleed. See pages 5-14 and 5-15 of the maintenance manual shows
the use of the J-23709 tool. That piece of information has been in our
Archives for 50yrs, who knew? That tool continues to be used on today's
available combination/proportioning valve. SEE:
https://techtalk.mpbrakes.com/proportioning-valve-bleeding-tool

The end result of this discussion is:
1. If you have a Cast iron distribution valve,......... care for it by
Flushing your brake system now and often. Moisture kills the Cast iron
sealing surfaces.

2. If you have a brass distribution(combination) valve fitted on your
coach,.... it is not as original equipment functionality and reduces rear
brake pressures significantly. In other applications the PV-2/PV4 valves
are appropriately modified as shown here:
https://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf
If you want cost effective braking improvement, just do it!

3 For those who have had issues with Brake bleeding get the bleeding tool

I am sure that many will comment that I have used Brass valves for the last
twenty years and have had good brakes and have been able to bleed them. "I
have always had good brakes." MY REPLY MUST NOW BE AS COMPARED TO
WHAT........SHOW ME THE DATA!

--
Regards,

Tom Pryor
4188 Limerick Dr
Lake Wales, Fl 33859
Cell 248 470 9186

1977 23'B named "CASPER", HARDLY ORIGINAL, (455 EFI) (Pwr. Drive)
(tailgate) (rear bunk beds)
(Webasto petrol boiler) (MB Elect fan clutch) (Brake reaction arms) BUT
STILL A WORK IN PROGRESS!

ReplyForward
 
Great thread!

I just removed the piston from my new combination valve while mounted. All went very well with the exception that the shuttle came out with the
piston. It actually made it easier to remove the piston while out I simply removed the piston and pushed the shuttle back in the combination valve. I
hope this didn't damage the switch? If no damage, all went extremely well.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
I have the original combination valve, 8,000 mile and stored indoor for over 25 years. Should I use the old valve or the new corrected valve?

Thanks,
Tom K.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
> So I was told that this valve is the only genuine and correct GMC brass combination valve that is New Old Stock and was the one to use.
>
> Was I mislead?
>
> https://www.highwaystars.net/brake-combination-valve-genuine-gm-1257177-for-gmc-motorhome-1974-sd-455-trans-am/

A definite maybe....Personally, I would not trust that it is correct until I took it apart and made sure that rear assembly was NOT pressure limiting.
Read SLIDE #7 of Dave Lenzi's "Brass Proportioning Valves" article in the Winter 2021 issue 154 on P.28 of GMC Vintage RVing. Or see the 154 issue at
this link. https://www.gmcmi.com/gmc-vintage-rving-2/

--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
> I have the original combination valve, 8,000 mile and stored indoor for over 25 years. Should I use the old valve or the new corrected valve?
>
> Thanks,
> Tom K.

Tom,

I can't think of a good reason to replace the existing and functioning part. The reason I did was that one of the supply lines from the master
cylinder did not re-seat right the second time I had to have the front frame out.

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
 
Thanks Matt,

I saved the old one. I will keep it in a zip lock bag. If I have and issue with the new one I will promptly switch it back. I very much appreciate
your and everyone's advice, tips and tricks.

Take care,
Tom K.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
The GM unit sold by Highway Stars is the best current solution today. Mr.Stone has been using this part number for over a decade ‘as is’ with
proportioning and zero failures. The MBM PV2 have had what I would very kindly call a high fail rate. At times the 2nd replacement still either
leaking or not passing pressure. Bob can elaborate on this. The end user can modify the GM brass valve per the CPP link and defeat the proportioning
so valve retains hold off, safety shuttle and light switch. Again, this is to be done by end user in a simple pre install procedure. Stock, the GM
brass unit when above threshold, reduces rear pressure as a PROPORTION of front pressure as drum brakes are self energizing and in most cases
untethered this leads to rear drum lockup before front disc lockup. Remember that Cinnabar installs MBM PV2s with proportioning as common practice.
The MBM PV2 has visually smaller fluid ports which may add milliseconds to wheel cyl fill time by acting as a fluid orifice. Scientific testing needs
to be done to time this. Bob has done pressure gauge testing on GM valve stock and with CPP mod completed and confirmed the modification does
eliminate proportioning.
The MBM PV2 is so prevalent because there is really no competition in the aftermarket. GM does not license reproduction brake parts due to liability.
So having a company build the OE TZE valve but in brass would have no endorsement possible by GM. The best bet I see is to do the CPP mod on the GM
old stock brass valve.
I would not trust an OE steel valve shuttle to function in a one sided fluid loss scenario. GM vehicles had steel combination valves from factory into
the 80s. However by 1990 GMSPO had replaced the steel with brass. I believe there were TSBs or recalls due to steel corrosion when DOT 3 became wet
saturated.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
John, thanks for addressing my question. Just so I am clear, in reading your reply it looks like my combination valve does have a proportioning
component, but it provides the pressure applied to the rear brakes as a predetermined fraction of pressure applied to the front discs, in order to
prevent premature rear wheel lock up.

Do I have this right?

If this is the case, it doesn't sound like the proportioning component is a bad thing.

Or am I wrong? Do I need to disassemble and modify mine?

After replacing every component in my brake system, changing to 80mm front calipers and 1-1/16" mid axle wheel cylinders, and bleeding with a Hupy
style pressure bleeder, I am still not impressed with braking performance

Thank you.
--
Greg Crawford
KM4ZCR
Knoxville, TN

"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
 
> John, thanks for addressing my question. Just so I am clear, in reading your reply it looks like my combination valve does have a proportioning
> component, but it provides the pressure applied to the rear brakes as a predetermined fraction of pressure applied to the front discs, in order to
> prevent premature rear wheel lock up.
>
> Do I have this right?
>
> If this is the case, it doesn't sound like the proportioning component is a bad thing.
>
> Or am I wrong? Do I need to disassemble and modify mine?
>
> After replacing every component in my brake system, changing to 80mm front calipers and 1-1/16" mid axle wheel cylinders, and bleeding with a Hupy
> style pressure bleeder, I am still not impressed with braking performance
>
> Thank you.

The way I see it, the GM Engineers decided that they wanted full pressure to the rear brakes for some reason; perhaps because unlike passenger cars
and pickup trucks, there is close to half of the weight of the vehicle over the rear wheels. It makes sense to me to have a combination valve that
does not have a proportioning section that limits pressure to the rear 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 disc brakes require more hydraulic pressure to operate than the drum
brakes do. That is why the engineers limited the pressure to the drum
brakes. When you have all discs on a coach, there needs to be the same
amount of pressure to front and rear, and no delay to the front brakes like
there is with a combination disc/drum system. Not too complex, really.
Jim Hupy
Salem, Oregon

On Sun, Jul 11, 2021, 9:24 PM Bill Van Vlack
wrote:

> > John, thanks for addressing my question. Just so I am clear, in reading
> your reply it looks like my combination valve does have a proportioning
> > component, but it provides the pressure applied to the rear brakes as a
> predetermined fraction of pressure applied to the front discs, in order to
> > prevent premature rear wheel lock up.
> >
> > Do I have this right?
> >
> > If this is the case, it doesn't sound like the proportioning component
> is a bad thing.
> >
> > Or am I wrong? Do I need to disassemble and modify mine?
> >
> > After replacing every component in my brake system, changing to 80mm
> front calipers and 1-1/16" mid axle wheel cylinders, and bleeding with a
> Hupy
> > style pressure bleeder, I am still not impressed with braking performance
> >
> > Thank you.
>
> The way I see it, the GM Engineers decided that they wanted full pressure
> to the rear brakes for some reason; perhaps because unlike passenger cars
> and pickup trucks, there is close to half of the weight of the vehicle
> over the rear wheels. It makes sense to me to have a combination valve that
> does not have a proportioning section that limits pressure to the rear
> 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.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
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