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From single to dual MC

716 Views 4 Replies 3 Participants Last post by  Scott302
What happens if I just replace current single-reservoir disc brake Master Cylinder with aluminium dual MC (it is from Baer kit, a friend did not use it in his late model) into existing brake lines without doing anything else. Except that new MC has two outlets as the current single-reservoir MC but on the right side. I am sure some brake shop can do the brake lines. I don’t have the tools. The bigger reservoir is for the front discs, isn't it?

1) I have 65 with disc brakes and single reservoir disc brake MC and rear dums. It has a distribution block for front brakes and a proportioning valve in the rear brake line. Would this be enough or is it mandatory to install a adjustable prop.valve?

2) How deep should the MC piston go when it is dry? I was trying measure the needed pushrod length. It seems that current pushrod for single-reservoir MC doesn’t move as much as the the piston in new MC would go. Do I need a new pushrod?

3) Also the new MC has piston diameter of 1 1/8 inch and I think that original has 1 inch. I am not planning to install brake booster. My legs aren’t the weakest, but is this setup too much.

Door handle first when cornering
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The brake proportioning valve on the 65/66 is an adjustable unit.As to master cylinder choice read this from "":
Formula for Master Cylinder Pressure

How do you determine the pressure output of the master cylinder. The following information will help you determine the proper size master cylinder:

To figure how much pressure your master cylinder is putting out:
C = pedal ratio
D = pounds of pressure apply by your foot
E = area of you master cylinder
F = pounds of pressure out of the master cylinder
C X D /(divided by) E = F

Example: If you have a 1" master cylinder the area equals 1/2" x 1/2" x 3.14 = 0.785 Square Inches. So, 100 pounds (of applied foot pressure) X 6 (pedal ratio) divided by 0.785 = 764 pounds of pressure.
If you have a 1-1/8" master cylinder, 100 psi X 6 (pedal ratio) divided by 0.9935 = 604 pounds of pressure.

Here is some info on master cylinder with "constant" of 6 to 1 pedal ratio and 100 psi being applied.
3/4" master cylinder = 1359 psi
7/8" master cylinder = 998 psi
15/16" master cylinder = 870 psi
1" master cylinder = 764 psi
1-1/8" master cylinder = 603 psi

Formula for Pedal Ratio:

Pedal ratio is the ratio of leverage you brake pedal applies to the master cylinder. To determine the pedal ratio you need to measure the height of the pedal to the pivot point then divided the measurement of the pivot point to the lower arm that controls your rod to the master cylinder.

A = height of pedal
B = center to center measurement of the lower arm
C = pedal ratio
A divided by B equals C
Or example 9" divided by 1.5" equal 6 to 1 ratio.

If you apply 100 pounds of pressure to the brake pedal, 100 pounds X (6 to 1) = 600 pounds of pressure. So, if the brake pedal has been modified from its' original design the pedal ratio is effected drastically. You can now see the pedal ratio is a "multiplier" of the pressure you apply with your foot, because this is the leverage that is applied to the master cylinder.

Now, take this same formula and substitute 2" instead of 1.5" you end up with a 4.5 to 1 ratio. Multiply 4.5 times the 100 pounds of applied pressure and you get 450 pounds instead of 600 pound. That half-inch cost you 25 percent of your braking power. The same thing applies when you shorten the upper measurement.

Greg B

· Registered
949 Posts
Ok, help me out....I keep putting the scale on top of the brake pedal
but when I try to do a hardstop from 60mph *and* read the scale at
the same time I keep hitting something! Are my feet too big?
<smiles and winks>

· Registered
3,046 Posts
Discussion Starter · #4 ·
Now, that's what I call an answer. It might take a weekend to figure it out, but I promise, I'll try. Thanks.

About the adjustability. Which way the adjuster turns? If I turn clockwise does it increase rear brake pressure. I think there was only something like "adjusting should not be tried" or some other threat in the Shop Manual. But when you say it is adjustable, it makes sense that I remember seeing threads in that proportioning valve.

Door handle first when cornering

· Registered
408 Posts
A product you might be interested in is a brake pressure gauge. It replaces the bleeder screw and allows you to read the pressure exerted on the cylinder or caliper. It is a great tuning aid for diagnosing brake problems or getting the bias adjusted correctly.

National Parts Depot
1965 Conv:Street driven concours
1966 Coupe: Daily driver and toy
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