Rick my manual Page 5-3, says that the METERING VALVE FUNCTION operates to
"hold off" hyraulic flow(pressure) until 130psi has been built up in the
system before applying the front brakes. that means there is no pressure
applied to either front or back brakes until 130psi is reached. This
function assures balanced braking during mild stops, and as you said
overcomes the rear spring tension. It also limits pressure to the rear
brakes to prevent lockup.
>In a message dated 3/4/99 6:04:01 AM MST, Patrick.Flowers
>writes:
>
>> 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). The reason is that the rear brakes have return
>springs, whereas the front discs don't. Hydraulic pressure has to overcome
>these springs and move the shoes into conatct with the drums before ANY
>braking occurs in back.
> Most foreign cars with disc/drum brakes don't use this valve, but the
>Americans found that without it, the front brakes wound up doing nearly ALL
>the work in normal driving, causing grossly premature pad wear. So, the
>metering valve (part of the combination valve on our GMCs) was introduced to
>share the wear.
> BTW, I assume that those with 6-wheel discs have disabled or bypassed the
>metering function of this valve, right?
> Gory details regarding the combination is found in the X-7525A shop manual,
>at the beginning of Section 5 (page 5-3 in my edition). HTH.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"hold off" hyraulic flow(pressure) until 130psi has been built up in the
system before applying the front brakes. that means there is no pressure
applied to either front or back brakes until 130psi is reached. This
function assures balanced braking during mild stops, and as you said
overcomes the rear spring tension. It also limits pressure to the rear
brakes to prevent lockup.
>In a message dated 3/4/99 6:04:01 AM MST, Patrick.Flowers
>writes:
>
>> 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). The reason is that the rear brakes have return
>springs, whereas the front discs don't. Hydraulic pressure has to overcome
>these springs and move the shoes into conatct with the drums before ANY
>braking occurs in back.
> Most foreign cars with disc/drum brakes don't use this valve, but the
>Americans found that without it, the front brakes wound up doing nearly ALL
>the work in normal driving, causing grossly premature pad wear. So, the
>metering valve (part of the combination valve on our GMCs) was introduced to
>share the wear.
> BTW, I assume that those with 6-wheel discs have disabled or bypassed the
>metering function of this valve, right?
> Gory details regarding the combination is found in the X-7525A shop manual,
>at the beginning of Section 5 (page 5-3 in my edition). HTH.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
