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  #1 (permalink)  
Old 11-18-2005, 09:54 AM
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need help on brake setup

We are going to replumb a 25 year old brake system that is real mushy.
It has no valves. So far, We have a new master cylinder, properly sized. We have disc fronts and drum rears. Based on research we need a metering valve (coordinate the disc/drum response) a proportioning valve (adjust front/rear lockup pressure) and residual valves for front and rear (2# front disc & 10 # rear drums) for the under floor master cylinder. Now comes the confusion. The local speed shop says "just use a proportion valve" Rod shops advertise "combination" valves that do "everything", yet the proportioning is not adjustable, and when I ask, they can't confirm that it has residual valves built in.
Can anyone tell me, for a fact, that a combination valve from ???????? is a metering valve, self adjusting proportioning valve and contains a 2# front and 10# rear residual valve, or not? If I install a combo valve that has a residual built in and I add a separate in line residual, does that screw things up?

BTW. We are building a 28 highboy with regular 15 inch front tires and 16x14 Mickey Thompsons on the rear, so I have big concerns about the disparity of front to rear weight and front to rear traction ratios.

thanks.

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Old 11-18-2005, 10:08 AM
STUPID is as STUPID does...
 
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Because of your tire size variation go with the individual components. You WILL have to adjust the proportioning valve to balance the brake bias and knowing exactly what you have for residuals is not a bad thing. Most disc/drum combination valves will have the residuals correct but, like you say, there is no adjustment for bias. It's better (in my mind) to know exactly what you have in a custom system as it really helps when it comes to trouble shooting.
Mark
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Old 11-18-2005, 07:02 PM
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Listen to astroracer.

I'll go ahead and address your questions anyway though
I haven't yet seen a combination valve which contained a residual valve, but I haven't checked them all. The Delco style one that MPbrakes advertises as having one for the rear, does not.

A Delco style disc/drum combination valve (the brass ones you see advertised everywhere) will have both metering and proportioning, but, as stated, it is preset, and may not suit your needs.

You dont really need a metering valve for the fronts except in certain situations, such as ice and snow, or if you ride the brakes. Under moderate and hard braking, it runs full flow anyway.

Running multiple residual valves will not multiply residual pressure. Residual pressure will defer to the strongest valve, and any behind it will be held open. Any weaker valve between the strongest and the master will hold its respective pressure in the line.

None of this really matters anyway because you should listen to astroracer and just run an adjustable to the rears, with seperate front/rear residual valves, due to the non-stock nature of the vehicle. Make sure you test the residual valves. If you decide you really still want metering, you can buy a seperate metering valve to delay the fronts. There is a 3way metering valve available (1line in, 2 out).

The main benefits of using the Delco style combination valve is that it has a warning switch, and shuts off flow to the failed circuit when under pressure. Some other type comb. valves will also bypass the metering or proportioning restrictor when the opposite circuit fails.

If it is not spec'd right for your needs, though, dont use it. IMO, all of these are factory failsafes to avoid costly returns and lawsuits. The proportioning section of these combination valves is preset for a range of vehicles to ensure the rear brakes never lock before the fronts, for any conceivable pavement conditions. The metering section is also spec'd depending on vehicle, but has a lot more leeway.
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Old 11-19-2005, 10:33 AM
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Thanks guys, I will follow your advice.
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