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#24 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,442
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Re: Brake problem question
The "Combination Valve" includes the "Metering Valve", "Pressure Differential Switch" and the "Proportioning Valve". Three valves in one. The "Metering Valve" is sometimes called a "Hold-Off Valve".
>>The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.<< It is only semantics, but to put it a different way, the "Hold-off Valve" prevents the front brakes from working until pressure reaches a certain point. The pressure point varies by vehicle, but let's say 50-100 psi. The rear drum brakes are unaffected by the hold-off valve. The rear brakes would act normally, not knowing if the hold-off was bad or even if there was one present. The proportioning valve doesn't play a role until pressure is upwards of 500 psi. The proportioning valve effects rear brakes only, and only above 500 psi. MC is leaking - bad MC Differential switch is ON - number one suspect at this point is bad MC. Bad nose dive sounds like hold-off valve is working and you have little or no rear brakes - bad MC. I would caution anyone from taking a combination valve apart in hopes of repairing it. If you damage any seals or valves, there are no replacement parts or kits.
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'67 GMC 2500, 292, 4spd, AC |
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