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  #31  
Old 05-03-2017, 04:48 PM
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simplyconnected simplyconnected is offline
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'Rule of thumb' is... The larger reservoir feeds disk brakes (because they have 2.5" (63mm) pistons/each).

Since dual master cylinders have the same diameter pistons (yes, there are two separate pistons but on the same shaft), if your reservoirs are the same size, it doesn't matter which one services the front.

For the same piston diameter reason, your brake light switch can be plumbed to either front or rear systems. It doesn't matter which one you use.

Truth of the matter is... Your combination valve FRONT ports, all three, are open to the M/C. (Check this out by blowing on the top front port as you feel both front exit ports.)

So, where does the metering come in for the rear system? Those huge front pistons take a lot of pressure to 'pinch the rotor' whilst the rear system has small pistons that very easy expands shoes.

So... you apply hydraulic pressure, both circuits get charged but the rear doesn't 'hold back' right away, causing the rear circuit to 'grab' and stop first. As more pressure is applied, a spring in the rear circuit is overcome and the valve starts proportioning which 'holds back' the rear circuit as the front pistons always advance with full pressure.

I hope this explanation helps. - Dave
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  #32  
Old 05-03-2017, 10:02 PM
stubbie stubbie is offline
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I'm not a brake expert but my understanding of how a dual master cylinder works is that. When you press the brake pedal that activates the rear brakes first. Once the shoes make contact with the drums that then activates the front brakes. This obviously all happens within seconds. The proportioning valve as Dave said Is there to regulate the pressure between front and rear to avoid rear brake lock up. As far as the switch goes I would not think it would make much of a difference as to wether it is on front or back.
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  #33  
Old 05-03-2017, 10:23 PM
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Phil, some of the OEM master cylinders had a valve in them for the rear but it was not a proportioning valve. They are not 'time-based' but rather, 'pressure-based'. Just like your accelerator pedal, you don't always mash the brake pedal down all the way, but you regulate your machine according to your needs. Aftermarket masters don't come with any valves.

Even so, OEM's still used proportioning valves if they had a disk/drum setup (which most of them did).

A combination valve does MUCH more. It:
Meters to the rear first,
Proportions the pressure (not the flow) between front/rear systems,
It has a 'center spool' that shifts between both systems, tying the hydraulics together, mechanically. <--this is important.

It also shuts off either the front or rear system in case a line ruptures or a wheel cylinder lets loose. When this happens, the electrical terminal grounds an "emergency ' light.

The system is good, and very reliable. I have it on two classics. - Dave
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