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need help with brake upgrade

702 Views 9 Replies 4 Participants Last post by  WILL
i need some help with changing master cylinders. i changed my rear brakes from 10" to 11" on a cj7. i followed a link somewhere a couple of days ago that said the master cylinder bore needed to be larger for the 11" brakes but i can't remember what the diameter was. what vehicle do i need to ask for a master cylinder for at the parts store? thanks in advance.

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Will, if the wheel cylinders on the 11" brakes are the same size as the ones on your old brakes there is no need to change mastercylinders.

don't fix it if it ain't broke. he's right in that if the internal size of the brake cylinder did not change, then you do not have to change masters. nothing in the hydraulic system has changed and therefore nothing new needs to be added. you should be able to find out the sizes from your parts guy.good luck, steve

Going to a larger bore master cylinder would defeat the whole purpose. The smaller the master
cylinder bore the more line presure. Let's say that you push with a hundered pounds of pressure on the
brake pedal and let's say you have an eight to one leverage on the cylinder, thats 800 pounds on the
cylinder. If the area of the piston is one square inch that's 800 psi but if the area of the piston is
increased to two square inches the pressure reduces to 400 psi. Of course this is for a single piston
manual master cylinder, but the same analysis applies to a power dual.

Decreasing the pressure could be overcome by increasing the size of the slave cylinders accordingly.
But, even if you increased the bore size of the rear cylinders the front didn't change and in normal use,
the front brakes do most of the braking. Therefore, increase the master cylinder bore will result in less
braking force.

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sounds like it is time to visit your friendly parts guy. if your 11in brakes use a larger wheel cylinder, then you must use a larger bore master cylinder to provide the volume of fluid needed to operate properly. a look at a raybestos master cylinder ap guide will tell you what wheel cylinders were used with that master.


I do not intend to offend,I offend with my intent
thanks for the replies. i found where i read about the master cylinder. it is in the january issue of four wheel drive and sport utility. it said when changing from 10" brakes to 11" brakes, the master cylinder needs to be changed from 1" bore to 1 1/16" bore. now i just need to find what vehicle came with the larger bore.

if i didn't have any problems with my brakes, i wouldn't be making any changes. since i swapped axles though, my rear brakes haven't been doing much. my front brakes only are causing a lot of nose dive. if anyone is interested, i'll post my findings when i change the master cylinder.


I believe the MC from a 1978 CJ has the larger bore you are looking for.
Ask to see one at your parts store and measure it.

you were right, jaffer. the 78 had 11" brakes as an option and uses a different master cylinder. i ordered one and it will be here tomorrow. thanks for the help.


That's what I was gonna say. I have a 78 with 11 inch brakes in the rear and discs up front. I would have also recommended a master
cylinder from a 78. But, you've done that already.

i put the larger bore master cylinder on last night. the back brakes work now so no more nose dive from just front brakes. thanks for all the help, i love this bbs.


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