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Discussion Starter #1
I'm just about to drop some change on a combination valve for my 68. It has 69 single piston front disks. The pistons are ~ 2 1/2" ( I could double check that). The rears are drum. I can't assume the rear brakes are stock since the rear-end is non-standard.

The combination valve is sold by MPBrakes - part number VL3350 for $99.95.

My question is this. Does a combination valve assume a fixed ratio of braking force such a stock disk / drum setup. In other words, can I buy a combination valve with the wrong proporting ratio if my hybrid brake system isn't matched to the valve?

If so, then I guess an adjustable proportioning valve and residual pressure valve are in order.

-dnult
 

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Stock 69 calipers are 2 15/16" bore size.
There are some 80's "Metric" calipers that have a 2 1/2" bore. If you have the smaller bore the prop valve for a 69 won't work well.

You may be able to buy a valve from one of the vendors that use the Metric caliper in their kit conversions, or use an adjustable one.
David
 

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Discussion Starter #4
I know what you mean Joe. But I hope it isn't too good to be true. My original question I guess leads to this. Will their combo valve (or proportioning valve) work with any setup or their setup? Seems like the combination of piston sizes, rotor / drum diameters, and weight distribution would require different *proportions*. I sure hope that isn't the case, because I like the idea of the combo valve without an adjustable valve. It makes you wonder if you install their valve with an adjustable prop. valve in line whether you can hit the target. In other words, will a proportioning valve in series with another (adjustable) proportioning valve put you outside of the adjustment window?

-dnult
 

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I see what you're getting at. Good questions. Combination valves are one of those tricky aspects of piecing together a system like brakes.

But if I understand what you're doing with your brakes correctly, it's the same as what I'm doing. I have to assume that the combi. valves being sold by the aftermarket as 'replacements' should have the same specs as factory, or very close.
And if I'm figuring the whole thing out right, I\we should be able to dump everything off of the braking system like the metering valve, idiot light block, residual valves for the rears and just plumb a combination valve under the master and be done with it. Again, assuming we're building the disc\drum system to factory specs.

Now, to your concerns. If you put the disc\drum system together with a combi. valve, factory single pistons and rotors, the brakes SHOULD work without locking the rear drums up prematurely. If not, and the rears need "proportioning", a 10lb residual valve or adjustable valve will have to be plumbed into the rear line.
 

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Discussion Starter #6
I've just about decided to go for it. From what I've learned is that the proportioning valve doesn't really proportion in the sense of attenuating pressure to the rear in proportion to the front pressure as I thought. Instead, the pressure rise in both front and rear are relatively eaqual until some "kneepoint" pressure at which the prop valve blocks off the rear circuit. Also, this happens only under panic braking. I learned this through a link on David Pozzi's web site.

So the way I figure it, is if I end up with a problem, I can try to modify the prop valve spring, or remove it alltogether and install an adjustable valve. In any event, I wont' be wasting my money, since I'll at least get a shiney new differential pressure switch with a metering valve for the front.


I managed to find an email address for mpbrakes and asked them this question. If they respond, I'll let you know what they say.

One other interesting tidbit. The article I was reading explained that since prop valves block off the rear under heaving braking, it can be a danger if the front circuit is lost. In other words, if you have to depend on the rears to stop the vehicle, they may not work. :eek: So it's good advice to ensure the emergency brake is working as it should.

-dnult
 

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i used the original valve (yep the square one) from the old drum setup and it works fine ..doesnt stop to fast or to slow (unless you hammer it then it stops ya pretty quick)...figured id try it first before getting new brake lines to match up with the roundish prop valve..someone told me the master does all the proportioning...i dont know for sure but it worked for me.
 
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