dual master cylinder

[V8COOPMAN]

Originally Posted by 1948F-1Pickup

The movement increase is about 3/8" at the master when comparing the larger bore master of 1 1/16" and the greater amount of piston movement with the smaller bore master of 1 inch. (0.866" piston travel in the 1 1/16" bore to move 1 cubic inch of brake fluid vs 1.27" piston travel in the 1" bore to move 1 cubic inch of brake fluid)

With that extra 3/8" AND the pedal ratio, you might very well bottom the brake pedal into the floorboard on some vehicles/applications.
It wasn't a problem on my '48 truck but it just underscores that you wouldn't know the true story unless you did the math.

That's interesting that you actually took the time to calculate the additional travel required of the M/C's piston to displace the necessary volume of brake fluid. My main concern was to make it obvious to folks that may not be familiar with what we're talking about here, as well as the POSSIBILITIES of what can happen if you're not aware.

The math that I am seeing here doesn't tell the ENTIRE story. If you subtract 0.866" from 1.27", that leaves an ADDITIONAL distance of 0.404" that the PISTON must travel. That is not the main concern. You must also take pedal travel into account. This means that the pedal pad (the part that your foot pushes on) must travel a GREATER distance which is dependent on the pedal ratio. Pedal ratio involves the difference in the distance of the pushrod pivot pin from the pedal-pivot centerline vs. the distance that the pedal pad is from the pedal-pivot centerline. If the pedal footpad is five (or more) times farther from the pedal pivot point, the pedal RATIO is 5:1. That means that the pedal PAD must be able to travel FIVE TIMES the ADDITIONAL DISTANCE that the piston must travel. In this case, we're talking about an additional 2.02" PEDAL TRAVEL.

BEWARE.....as this does NOT INCLUDE any additional pedal travel necessary as I referenced ABOVE because of a leak in one side of the system. All that I am suggesting here is that it should be wise to induce a "test leak" in each side of your newly-plumbed brake system to insure that you still have full-stroke-pedal capabilities should a leak develop in either side of your system. Also beware that "full stroke" means WITHOUT THE M/C BOTTOMING-OUT. The M/C should NOT bottom-out.

Coop


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