Master Cylinder size for disc brakes

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Master Cylinder size for disc brakes

Postby MogDJ » Tue Oct 17, 2017 7:59 pm

Hi Guys,
I'm in the process of fitting a disc brake conversion to my Pick Up, and have just removed the Master Cylinder to remove the small seal. There is no size on the MC, but measuring the main cup seal it looks like it's a 7/8" one. I'm aware that later moggies had a 13/16" MC fitted on standard brake set ups, and since I've taken the MC out, I was wondering which size of MC would be most suitable for the disc brake conversion, as I'm led to believe that the disc brakes require more fluid to operate correctly!

Any advice or info would be very welcome.

Ta very much.
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either is ok,

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Re: Master Cylinder size for disc brakes

Postby emjay » Wed Oct 18, 2017 3:29 pm

Bear in mind the smaller the bore will have higher fluid pressure but a longer stroke for a given wheel cylinder displacement volume and pedal effort. Now if the disc brakes you have have larger pistons, they will require a larger volume of fluid to move the pads the same amount. Once everything is installed, establish where your pedal is when fully depressed and then drive it and evaluate the braking performance and how much pedal force is needed to achieve it.
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Re: Master Cylinder size for disc brakes

Postby MogDJ » Wed Oct 18, 2017 5:18 pm

Thanks guys,
I now have a dilemma, I have both sizes of Master Cylinder in excellent condition, neither has been used as I've been fully rebuilding the pick up for quite a few years, and bought the wrong size ( the 7/8" ) early in the rebuild and changed it when I realised I had the incorrect unit!
So which one should I fit to try the brakes as suggested by emjay. I would prefer not to have to change one later if at all possible! Haha.
I understand that this is a difficult call to make, but would welcome any views any of you may have, which at least would allow me to make an educated guess.

Many thanks.
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Re: Master Cylinder size for disc brakes

Postby emjay » Wed Oct 18, 2017 5:26 pm

Which disc brakes are you installing? Then others who have installed the same can relate their results. What do you have on the rear?
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Re: Master Cylinder size for disc brakes

Postby MogDJ » Wed Oct 18, 2017 6:41 pm

I'm fitting the "grumpy" conversion, based on the Marina set up, purchased at ESM Minors. The rear brakes are the standard Minor set up.
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as above either, you will not notice any difference if using a servo(preferred with marina system)

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Re: Master Cylinder size for disc brakes

Postby MogDJ » Thu Oct 19, 2017 4:02 pm

Thanks Woodywoodchipper, I think I'll go with the 7/8"" one which is a Lockheed unit rather than the 13/16" one which is a pattern one. I hadn't planned on fitting a servo to be honest, but may need to consider one if the braking isn't good.
I presume that it will still work without one, but pedal pressure may be affected?
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Re: Master Cylinder size for disc brakes

Postby geezer » Fri Oct 20, 2017 12:36 pm

On my conversion I used the 13/16" master. The front caliper piston is 54mm for my conversion. Pedal travel is just right. I would also suggest a 10lb residual check valve in the rear brake circuit and 2lb residual check valve for the front. That will give a better pedal feel and shorten the pedal travel. When I went to 4 wheel discs I replaced the 10b in the rear to 2lb. Before adding the check valves the pedal travel was a little longer than I cared for.

http://www.morrisminorforum.com/journal/geezer/13365

http://www.morrisminorforum.com/journal/geezer/13445
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MogDJ wrote:Thanks Woodywoodchipper, I think I'll go with the 7/8"" one which is a Lockheed unit rather than the 13/16" one which is a pattern one. I hadn't planned on fitting a servo to be honest, but may need to consider one if the braking isn't good.
I presume that it will still work without one, but pedal pressure may be affected?


you will find the pedal very hard to push, yes it will stop but will be like good working drums,

fit a servo it will transform it, ;)
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Re: Master Cylinder size for disc brakes

Postby ruairidh » Fri Oct 20, 2017 9:06 pm

Indeed servo and a brake reservoir kit which lets you put the brake fluid under the bonnet and seal of the mc. :)
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