Master Cylinder Question - Help

[Ralph Z.]

Have a 1968 Alexis MK14 Formula Ford. The car came with Girling 12SP calipers on the front and Girling 14LF calipers on the rear.

We have changed out the front calipers with 14LF so we now have the same 14LF calipers on all corners. The 14LF calipers have larger diameter pistons than the 12SPs.

However, the master cylinder for the rear is 3/4 diameter and the master cylinder for the front it .670 diameter.

What size master cylinders should be used in order to have appropriate break bias with all four corners now having the same calipers? The car has a cockpit adjustable brake bias adjuster.

Should the master cyliners be swapped so the smaller unit is on the rear and the larger on the front? Or, something else?

Thanks for your insights. We're getting ready for Mid Ohio later this month.

[Steve Demeter]

The smaller master will produce higher line pressure with a given force. With all 4 calipers the same you want inherently higher line pressure to the front. I would put the smaller master on the fronts. but remember that the combination of MC's that you have might not give the proper inherent balance. It might be off too much one way or another to tweak out with the bias bar..

Try the jack-stand method of checking brake balance in Carroll Smith's book. Basically have someone hold steadily increasing pressure on the pedal until by grabbing and turning a front tire until it just locks. Have them maintain that pressure. The rear should be just barely not locked. If you can not achieve that I would suggest different combination of master cylinders.

[roadracer]

Ralph,
A guess that the .670" master cylinder is really a .625" master cylinder on the front?
You should be fine with a .625" master cylinder on the front and a .750" master cylinder on the rear with brake calipers the same size front and rear.
With the mechanical adjustment you have there will be a minor change from where you started with the F/R different sized calipers. Not sure what brake caliper dimensions you now have. Typical sizes are 1.625" and 1.750".
Thanks!

[Ralph Z.]

Ralph,
A guess that the .670" master cylinder is really a .625" master cylinder on the front?
You should be fine with a .625" master cylinder on the front and a .750" master cylinder on the rear with brake calipers the same size front and rear.
With the mechanical adjustment you have there will be a minor change from where you started with the F/R different sized calipers. Not sure what brake caliper dimensions you now have. Typical sizes are 1.625" and 1.750".
Thanks!

Yes, may have made a mistake in reading it on the car. Probably a .625.

Caliper pistons on the 14LFs are 1 7/8".

[roadracer]

Ralph,
With your 'old' set-up brake bias (50/50 pedal bias) using Girling 12SP (1.6875") calipers on the front with 5/8" master cylinder and Girling 14LF Rear (1.875") with 3/4"rear master cylinder the balance works out to 55% front, 45% rear.

With the 'new' set-up using the same front and rear calipers and 5/8" front master and 3/4" rear master cylinder, using the same 50/50 pedal bias) you will end up with 60% front and 40% rear. Your mechanical bias adjustment should be fine to get back to your 55% front and 45% rear.

Good luck with this!

[Garey Guzman]

My 1971 March came to me with 14LFs on all 4 corners and the same 3/4 rear mc. My front mc is 5/8 so if that's what you have, it should be fine.

[mark elder]

Roadracer

You said

Ralph,
With your 'old' set-up brake bias (50/50 pedal bias) using Girling 12SP (1.6875") calipers on the front with 5/8" master cylinder and Girling 14LF Rear (1.875") with 3/4"rear master cylinder the balance works out to 55% front, 45% rear.

With the 'new' set-up using the same front and rear calipers and 5/8" front master and 3/4" rear master cylinder, using the same 50/50 pedal bias) you will end up with 60% front and 40% rear. Your mechanical bias adjustment should be fine to get back to your 55% front and 45% rear.

Are you able to help me through the maths on this one? I am having brain fade (and I suspect the 12SP might not be 1.6875).

Thanks

Mark

[roadracer]

Mark,
If the information is correct (Pegasus Auto Racing) the Girling 12SP caliper has pistons with 1 11/16" bore or 1.6875". Moving from a smaller caliper bore (Girling 12SP) to the larger (Girling 14LF; 1.875") will result in slightly less - 20% - rear brake pressure with the same pedal pressure. So the F/R adjustment will be off a bit from where Ralph started.
As stated in a previous post the different master cylinder sizes will compensate for the F/R ratio.
Hope this helps!

[Ralph Z.]

Thanks for all the input. Very helpful in thinking this through. We'll start with .625 MC on the front and .75 on the rear. I have in inventory a .70 MC if needed.

Thanks again for your help.

[Ralph Z.]

With the 12SPs on the front, the brake bias adjustment needed to be full forward in order to balance. Which, in actuality, left no adjustment.

With 14LFs on all corners, we should see the bias adjustment move closer to center.