Dual Master Cylinder Questions

Ok so I'm replacing my existing single circuit master cylinder with a pair of Wilwood master cylinders. So far so good but here's where I run into some problems. The existing master cylinder has a meassured bore of 0.72". That's close enough to 3/4" for me. I have a pair of 3/4" Wilwood masters from a previous project, if I use these masters will my brake effort double since I'm doubling the volume?
This set up is being fit into a Lotus Europa so there's limited pedal box real estate. To maximize my available mechanical advantage I may be forced into going to a hanging pedal design with an adjustable bias bar but this means more money and complexity.

If I were to install an adjustable brake pressure valve in the rear brake line would this work to adjust my front to rear brake bias or would it also increase my pedal effort?

I just ran this past one of my mechanical engineer counterparts, here at work, and he did an excellent job of impersonating a trout gasping for breath on a dock.

Volume should have nothing to do with brake effort.

In a tandem master cylinder, you are still pushing two pistons - the primary piston and the secondary piston - the pressure built up under the primary piston applies the secondary piston.

You can buy just the balance bar for about $60, I think. Just make your own pedals and assembly. The balance bar hole will shrink when you weld it - I resized mine with a brake cylinder hone.

An adjustable proportioning valve should not increase brake effort, it merely cuts off maximum pressure to the rear brakes.

If you're going to the effort to build dual masters, do a quick search for a spreadsheet called mikesbrakes.xls - it is really helpful in determining ideal brake master sizes. I used it for my Locost, and final setup was almost perfect right out of the box. I can email it to you if you like. g_wellwood AT yahoo DOT comorsomethinglikethat.

Yes, just size each of the masters like you would a single one. So 0.75" it is.

If you have a balance bar you don't need a proportioning valve. If the bar won't give you enough adjustment, then you can stagger the size of the masters. That's how my Seven is set up.

I can see how an adjustable valve would increase effort - not all of your pedal pressure actually gets to the rear brakes. But since that's simply to keep the brakes from locking, it's no loss. But go with the bar.

Thanks for the input but just to reiterate. The original master was a 3/4" single circuit not a tandem. Therefore there was only one single 3/4" piston working both front and rear brakes.

I'll go with a balance bar and fabricate my own pedal assembly.

keith, you're slipping. he's replacing a single 3/4" with parallel 3/4"s, so pedal effort will double but pedal travel will be cut in half.

to the OP, a single 3/4" fed all four brakes, right? that was 0.442 square inches of piston area. to get the same piston area with two parallel bores, the ideal bore would be 0.53". they probably don't make a 9/16" (0.563"). so your choices are probably 5/8" (0.625") or 1/2" (0.500").

using a pair of 5/8" will increase pedal effort by 40%, and will decrease pedal travel by the same.

using a pair of 1/2" will decrease pedal effort by 11% and will increase pedal travel by the same.

AngryCorvair wrote: keith, you're slipping. he's replacing a single 3/4" with *parallel* 3/4"s, so pedal effort will double but pedal travel will be cut in half. to the OP, a single 3/4" fed all four brakes, right? that was 0.442 square inches of piston area. to get the same piston area with two parallel bores, the ideal bore would be 0.53". they probably don't make a 9/16" (0.563"). so your choices are probably 5/8" (0.625") or 1/2" (0.500"). using a pair of 5/8" will increase pedal effort by 40%, and will decrease pedal travel by the same. using a pair of 1/2" will decrease pedal effort by 11% and will increase pedal travel by the same.

This is what I was afraid of. I have the pair of 3/4" masters already so the Grassroots way to go would be to increase the fulcrum length of my brake pedal. This would increase the pedal travel and bring the effort into a more realistic range.........I think.

You're right, I missed the reference to a single circuit. Most people view a dual-circuit tandem master as a single master, and I made the assumption that this was the case. Bad Keith.

It doesn't matter if your mechanical advantage comes from the pedal ratio or the hydraulics, the end result is the same. On the Westfields, we calculated we needed a 0.50" master on the clutch. Since we couldn't easily find one, we moved the position of the rod on the pedal to allow us to keep the existing master.

Thanks Keith and Merry Christmas from your frozen homeland.

what is this going into? If you are running a single circut 3/4 mc.. I can assume four wheel drum brakes?

Drums require less effort and fluid than discs. Also pedal effort is also a product of slave cylinder size. larger MC and a smaller bore SC means more effort, but less movement.. smaller MC and larger SL means less effort, but more movement.

It's a 1969 Lotus Europa S2. It has front discs and rear drums.