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Problem with brass body proportioning valve

T
tomkatz3@comcast.net
Sun, Jul 11, 2021 12:38 AM

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.

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.
T
tomkatz3@comcast.net
Sun, Jul 11, 2021 12:40 AM

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.

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.
L
Larry
Sun, Jul 11, 2021 1:24 AM

Greg C. wrote on Sat, 10 July 2021 19:05

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.

Greg C. wrote on Sat, 10 July 2021 19:05 > 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.
MC
Matt Colie
Sun, Jul 11, 2021 1:15 PM

Tom Katzenberger wrote on Sat, 10 July 2021 20:40

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

Tom Katzenberger wrote on Sat, 10 July 2021 20:40 > 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
T
tomkatz3@comcast.net
Sun, Jul 11, 2021 4:16 PM

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.

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.
JR
John R. Lebetski
Sun, Jul 11, 2021 6:33 PM

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

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
GC
Greg Crawford
Mon, Jul 12, 2021 12:29 AM

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. -- Greg Crawford KM4ZCR Knoxville, TN "Ruby Sue" 1977 Royale Rear Bath 403 Engine American Eagle Wheels Early Version Alex Sirum Quad bags
BV
Bill Van Vlack
Mon, Jul 12, 2021 4:24 AM

Greg C. wrote on Sun, 11 July 2021 19:29

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.

Greg C. wrote on Sun, 11 July 2021 19:29 > 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.
JH
James Hupy
Mon, Jul 12, 2021 4:33 AM

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 bill.van.vlack@gmail.com
wrote:

Greg C. wrote on Sun, 11 July 2021 19:29

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.


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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 <bill.van.vlack@gmail.com> wrote: > Greg C. wrote on Sun, 11 July 2021 19:29 > > 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: >
BV
Bill Van Vlack
Mon, Jul 12, 2021 3:34 PM

James Hupy wrote on Sun, 11 July 2021 23:33

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

That makes sense, but it gets more complex when trying to understand why the GMC Motorhome engineers did not do it that way. If I understand Dave
Lenzi's article correctly, the GMCMH disk/drum coach came with a steel combination valve that provided the same amount of pressure to the front and
rear brakes (after the front brake delay).

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.

James Hupy wrote on Sun, 11 July 2021 23:33 > 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 That makes sense, but it gets more complex when trying to understand why the GMC Motorhome engineers did not do it that way. If I understand Dave Lenzi's article correctly, the GMCMH disk/drum coach came with a steel combination valve that provided the same amount of pressure to the front and rear brakes (after the front brake delay). -- 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.