Brake Booster => Combo Valve

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> In a message dated 3/4/99 6:04:01 AM MST,
> Patrick.Flowers

>
> > The combination
> > valve on our GMCs does not allow any pressure on the rear until the
> > front brakes are up to a preset pressure(90psi).
>
> In response to Scott Adohen's query..
>
> >> It is the valve
> >> that causes a
> >> differential in hydraulic pressure exerted on the rear brakes
> >> so as not to
> >> have our front brakes grab first. Something like that? Right?
> >>
> >> Scott Adohen
>
> Patrick & Scott,
> Actually, Scott had it right. The combination valve does not allow
> pressure to the FRONT brakes until the REARS are up to
> pressure, (130 psi
> according to the manual).

Rick, I posted a note right after the one you quote recognizing that I
had stated it "bass-ackwards". I should never try to get technical
before the second cup of coffee.

> BTW, I assume that those with 6-wheel discs have disabled
> or bypassed the
> metering function of this valve, right?

Nope, you retain all the combi valve function. You still want the rears to
apply first. There's a lot of mass following the front wheels and nasty
things could happen if the front brakes were allowed to apply first(or even
simultaneously). It's like riding a motorcycle - you lead with the rear
brake
and then apply the front, keeping the pointy end in front and the blunt end
following.

Just having my second cup,
Patrick
 
I installed six wheel disc brakes in 1995 and eliminated the combination
valve in 1996. Many miles since then in all types of weather including snow
plus towing, and can't tell any difference!

Paul Bartz

From: Patrick.Flowers
[mailto:Patrick.Flowers]
Sent: Friday, March 05, 1999 8:09 AM
Subject: RE: GMC: Brake Booster => Combo Valve

In a message dated 3/4/99 6:04:01 AM MST,

The combination valve on our GMCs does not allow any pressure on the rear
until the front brakes are up to a preset pressure(90psi).

In response to Scott Adohen's query..

It is the valve that causes a differential in hydraulic pressure exerted on
the rear brakes so as not to have our front brakes grab first. Something
like that? Right?

Scott Adohen

Patrick & Scott,
Actually, Scott had it right. The combination valve does not allow pressure
to the FRONT brakes until the REARS are up to pressure, (130 psi according
to the manual).

Rick, I posted a note right after the one you quote recognizing that I had
stated it "bass-ackwards". I should never try to get technical before the
second cup of coffee.

BTW, I assume that those with 6-wheel discs have disabled or bypassed
the metering function of this valve, right?

Nope, you retain all the combi valve function. You still want the rears to
apply first. There's a lot of mass following the front wheels and nasty
things could happen if the front brakes were allowed to apply first(or even
simultaneously). It's like riding a motorcycle - you lead with the rear
brake and then apply the front, keeping the pointy end in front and the
blunt end following.

Just having my second cup,
Patrick
 
Interesting! Does Leigh provide the parts to eliminate the
valve? What's involved? I didn't think to ask him about
that at Marion - of course, it's obvious I thought that the
combi valve was retained.

Patrick

> -----Original Message-----
> From: Bartz, Paul [mailto:s9d3452]
> Sent: Friday, March 05, 1999 9:23 AM
> To: 'gmcmotorhome'
> Subject: RE: GMC: Brake Booster => Combo Valve
>
>
> I installed six wheel disc brakes in 1995 and eliminated the
> combination
> valve in 1996. Many miles since then in all types of weather
> including snow
> plus towing, and can't tell any difference!
>
> Paul Bartz
 
Patrick:

All I did was disconnect the lines from the valve, straighten the bends,
unbolt and remove the valve bracket from the frame, and reconnect the lines
(front with a t'ee and the rear circuit with a coupling, fittings). Don't
believe it even required any further bleeding either.

At the time I eliminated the valve, Leigh didn't have any parts and I found
out there is a difference in the fitting sizes between my 78 and his 73
coach's.

Paul

From: Patrick.Flowers
[mailto:Patrick.Flowers]
Sent: Friday, March 05, 1999 9:31 AM
Subject: RE: GMC: Brake Booster => Combo Valve

Interesting! Does Leigh provide the parts to eliminate the valve? What's
involved? I didn't think to ask him about that at Marion - of course, it's
obvious I thought that the combi valve was retained.

From: Bartz, Paul [mailto:s9d3452]
Sent: Friday, March 05, 1999 9:23 AM
Subject: RE: GMC: Brake Booster => Combo Valve

I installed six wheel disc brakes in 1995 and eliminated the combination
valve in 1996. Many miles since then in all types of weather including snow
plus towing, and can't tell any difference!
 
On the newer systems they use a two part master cylinder so if one section
goes bad the other still works. the front and rear brakes are independently
connected.

>I think by removing the function of the combination valve even though you
>change to 6 wheel discs, it should not be done. I think you would want the
>rear brakes to come on first before pressure to front brakes. I think you also
>lose the brake warning light. Further, I don't know if our combination valves
>also work like the newer systems work. In newer systems, if you lose pressure
>to some section of the brake system because of a leak, automatically the
>combination valve closes, so you still have some functioning brakes, albeit
>only half the system.
>
>In a message dated 3/5/99 9:40:06 AM Eastern Standard Time,

>
>> Patrick:
>>
>> All I did was disconnect the lines from the valve, straighten the bends,
>> unbolt and remove the valve bracket from the frame, and reconnect the lines
>> (front with a t'ee and the rear circuit with a coupling, fittings). Don't
>> believe it even required any further bleeding either.
>>
>> At the time I eliminated the valve, Leigh didn't have any parts and I found
>> out there is a difference in the fitting sizes between my 78 and his 73
>> coach's.
>>
>> Paul
>>
>> From: Patrick.Flowers
>> [mailto:Patrick.Flowers]
>> Sent: Friday, March 05, 1999 9:31 AM
>> Subject: RE: GMC: Brake Booster => Combo Valve
>>
>> Interesting! Does Leigh provide the parts to eliminate the valve? What's
>> involved? I didn't think to ask him about that at Marion - of course, it's
>> obvious I thought that the combi valve was retained.
>>
>> From: Bartz, Paul [mailto:s9d3452]
>> Sent: Friday, March 05, 1999 9:23 AM
>> Subject: RE: GMC: Brake Booster => Combo Valve
>>
>> I installed six wheel disc brakes in 1995 and eliminated the combination
>> valve in 1996. Many miles since then in all types of weather including
>snow
>> plus towing, and can't tell any difference!
>>
>>
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
 
Heinz
Did you eliminate your combination valve? When I talked to TSM, he
said there is no reason to eliminate it.
Just asking since we have same disc mod's.
herm

>
> I think by removing the function of the combination valve even
though you
> change to 6 wheel discs, it should not be done. I think you would
want the
> rear brakes to come on first before pressure to front brakes. I
think you also
> lose the brake warning light. Further, I don't know if our
combination valves
> also work like the newer systems work. In newer systems, if you
lose pressure
> to some section of the brake system because of a leak, automatically
the
> combination valve closes, so you still have some functioning brakes,
albeit
> only half the system.
>
> In a message dated 3/5/99 9:40:06 AM Eastern Standard Time,

>
> > Patrick:
> >
> > All I did was disconnect the lines from the valve, straighten the
bends,
> > unbolt and remove the valve bracket from the frame, and reconnect
the lines
> > (front with a t'ee and the rear circuit with a coupling,
fittings). Don't
> > believe it even required any further bleeding either.
> >
> > At the time I eliminated the valve, Leigh didn't have any parts
and I found
> > out there is a difference in the fitting sizes between my 78 and
his 73
> > coach's.
> >
> > Paul
> >
> > From: Patrick.Flowers
> > [mailto:Patrick.Flowers]
> > Sent: Friday, March 05, 1999 9:31 AM
> > Subject: RE: GMC: Brake Booster => Combo Valve
> >
> > Interesting! Does Leigh provide the parts to eliminate the
valve? What's
> > involved? I didn't think to ask him about that at Marion - of
course, it's
> > obvious I thought that the combi valve was retained.
> >
> > From: Bartz, Paul [mailto:s9d3452]
> > Sent: Friday, March 05, 1999 9:23 AM
> > Subject: RE: GMC: Brake Booster => Combo Valve
> >
> > I installed six wheel disc brakes in 1995 and eliminated the
combination
> > valve in 1996. Many miles since then in all types of weather
including
> snow
> > plus towing, and can't tell any difference!
> >
> >
>
>

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No. My understanding is that the combination valve also has an internal
valve that senses a pressure differenetial and closes the port that has the
pressure loss (if it is more than 300psi below operating pressure).

>In a message dated 3/5/99 12:07:24 PM Eastern Standard Time, warner
>writes:
>
>> On the newer systems they use a two part master cylinder so if one section
>> goes bad the other still works. the front and rear brakes are
>independently
>> connected.
>>
>
>So therefore, I'm NOT correct in thinking or hoping that we have this
>independence. If we get a leak somewhere, we have no brakes except for cable
>driven rear emergency brakes, right? If yes/ that sucks! If yes/ can
>anything be done to upgrade to newer two part master cylinder?
>
>Scott Adohen
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
 
OK, confession time. When the rear brake line rusted through
on my 73, I pinched off the bad line and drove the coach to
the shop(only about 2 miles). I had brakes, but not much.
The line that rusted through was the main line running along
the street side frame rail, so the rear brakes were completely
inop.

>From my experience, I'd say there are definitely two separate
systems with the combi valve.

Patrick

> -----Original Message-----
> From: Adohen [mailto:Adohen]
> Sent: Friday, March 05, 1999 12:55 PM
> To: gmcmotorhome
> Subject: Re: GMC: Brake Booster => Combo Valve
>
>
> In a message dated 3/5/99 12:41:14 PM Eastern Standard Time,
> warner

>
> > No. My understanding is that the combination valve also
> has an internal
> > valve that senses a pressure differenetial and closes the
> port that has the
> > pressure loss (if it is more than 300psi below operating pressure).
> >
> So until we hear differently, I will concluded that with a properly
> functioning combination valve, if we do get a leak in a brake
> line( other than
> leaks in lines from master to combination valve), we have 1/2
> hydraulic brakes
> somewhere with a lot lower brake pedal.
>
> Scott Adohen
>
 
Scott,

After ruminating on this all weekend, I gotta agree with you.
Recalling my experience with motorcycles(the only other vehicle
I can think of that has anywhere near the percentage of mass
following the front wheels as the GMC), I prefer to keep the
rear brakes coming in first.

That's my choice,
Patrick

> From: Adohen [mailto:Adohen]
> Sent: Friday, March 05, 1999 10:41 AM
> To: gmcmotorhome
> Subject: Re: GMC: Brake Booster => Combo Valve
>
>
> I think by removing the function of the combination valve
> even though you
> change to 6 wheel discs, it should not be done. I think you
> would want the
> rear brakes to come on first before pressure to front brakes.
 
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