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Old 09-10-2010, 09:23 AM
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Disc/Drum Setup - Combination Valve Necessary?

Hello Folks,

In a previous post I detailed the brake problems on my 1947 Plymouth. My power brake booster was defective, I replaced it, and the brakes are now much better.

To review, the Ply has front discs and rear drums off of a 1972 Nova, utilizing a Corvette-style MC and 8" dual-diaphragm booster under the floor, with a 2lb residual valve on the fronts and a 10lb residual valve on the rears. I do NOT have a combination valve, or any other sort of proportioning valve, installed in the system.

Although the brakes are much improved, it feels like the rear drum brakes are doing more of the stopping than the front discs.

Do I need to install a combo valve? If so, could I just use a VL3350 from MPB?

Thanks in advance.

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Old 09-10-2010, 11:24 AM
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It sounds like an adjustable proportioning valve is all that you need to give more front bias. But if the brake system is more out of balance than just a prop valve will deal with, the combo valve you mentioned will be fine.
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Old 09-10-2010, 12:54 PM
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Quote:
Originally Posted by cobalt327
It sounds like an adjustable proportioning valve is all that you need to give more front bias. But if the brake system is more out of balance than just a prop valve will deal with, the combo valve you mentioned will be fine.
Um, a combo valve and a proportioning valve do the same thing. The combination valve is a combination of the proportioning valve, distribution block, and differential pressure switch all in one package, hence the term "combination" valve.
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Old 09-10-2010, 03:43 PM
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Quote:
Originally Posted by joe_padavano
Um, a combo valve and a proportioning valve do the same thing. The combination valve is a combination of the proportioning valve, distribution block, and differential pressure switch all in one package, hence the term "combination" valve.
Saying they "do the same thing" is quite a stretch. A combo valve can have metering and proportioning functions as well as being a distribution point and contains a brake circuit pressure switch.

An adjustable proportioning valve is just that- an adjustable proportioning valve- period. A combo valve is incapable of the adjustability function.
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Old 09-10-2010, 08:40 PM
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Quote:
Originally Posted by cobalt327
Saying they "do the same thing" is quite a stretch. A combo valve can have metering and proportioning functions as well as being a distribution point and contains a brake circuit pressure switch.

An adjustable proportioning valve is just that- an adjustable proportioning valve- period. A combo valve is incapable of the adjustability function.
A proportioning valve is a proportioning valve. All it does is create a differential pressure in one side of the system vs. the other side. The incorporation of the distribution block or differential pressure switch is irrelevant and doesn't change how or what the prop valve does. Yes, the adjustable one will allow you to change that pressure differential, but I'm at a loss to understand how going from an adjustable prop valve to a factory-style combo valve with fixed proportioning will be an improvement.
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Old 09-11-2010, 01:57 AM
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Hows about this: Instead of debating w/ME on the pros and cons, offer up a suggestion to the OP on what YOU think he should do.
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Old 09-11-2010, 06:43 AM
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there ya go .
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Old 09-11-2010, 07:59 AM
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Quote:
Originally Posted by cobalt327
Hows about this: Instead of debating w/ME on the pros and cons, offer up a suggestion to the OP on what YOU think he should do.
Fair enough. I actually agree with your original response - the OP needs a proportioning valve. I'd use an adjustable one since the car is not a stock combination of parts.
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Old 09-11-2010, 08:24 AM
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I would start with a pressure check at the wheels to determine if the pressures look right for the front and rear system, then you have a better idea of what else may be needed on the lines. Corvette MCs were made for both disc/disc and disc/drum systems depending on the year. If a disc/drum MC is being used, are the MC output ports connected to the correct axles?

Just as an aside, an early Plymouth would seem to be heavier (maybe significantly heavier) than a Nova - it makes me wonder if '72 Nova brakes are adequate, even with a booster.
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Old 09-11-2010, 09:51 AM
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Quote:
Originally Posted by cucumber1949
I would start with a pressure check at the wheels to determine if the pressures look right for the front and rear system, then you have a better idea of what else may be needed on the lines. Corvette MCs were made for both disc/disc and disc/drum systems depending on the year. If a disc/drum MC is being used, are the MC output ports connected to the correct axles?

Just as an aside, an early Plymouth would seem to be heavier (maybe significantly heavier) than a Nova - it makes me wonder if '72 Nova brakes are adequate, even with a booster.
Point well taken about the weight of the Ply vs. the Nova disks - I've though about that myself.
FWIW the previous owner had this setup for 20 years with no complaints; whether that speaks to the system's adequacy - or his strong feet - remains to be seen.
However, before going forward with any sort of wholesale change, I'd rather optimize what I have first.

Oh, and BTW, Corvettes were all drums up until 1964, and all discs after that. They have never mixed discs & drums on the same vehicle.

Cheers, Ken
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Old 09-11-2010, 04:56 PM
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Quote:
Originally Posted by kbuhagiar
Oh, and BTW, Corvettes were all drums up until 1964, and all discs after that. They have never mixed discs & drums on the same vehicle.
My BAD!! You are right about how they came from the factory. We have a 64 in our condo garage with discs on the front, and I made an assumption that is how it came from the factory - obviously rather some kind of retrofit. Wonder how it handles. I learned something today.
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