I bought a 1969 Mini Mk2 Cooper S partially rebuilt. The owner said the master cylinders had been rebuilt. How do you tell which master cylinder is which or doesn't it matter.

There is a non return valve in the pressure outlet. Try to insert a wire into the outlet and you should see the difference.

Also its preferred to use 0.7" for the brake master to provide more braking pressure and 0.75" for the clutch master as it moves more fluid.

Brake MC is 0.70" bore. And may have a restrictor valve in top.
Some plastic clutch MCs were 0.70 but 0.75 preferred.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The bore size maybe cast into the side.

I think they are after market as there are no marking anywhere.

Tried the wire test, there does seem to be more resistance in one. Is the more resistance the brake MC?

Yes. The one with a non return valve will prevent the wire from going in (with light pressure). If you are not 100% sure just pull them down and check the internals. You can also check the bore size as well.

My 1970 Mini came with one Girlock and one Lockheed cylinder, both 0.7" bore. The Lockheed brake one being cast Aluminium alloy with the non-return valve and the Girlock clutch one cast iron. The cylinders looked awful but on inspection, after dismantling, the bores were in good condition and both had been resleeved. They hadn't been stored very well as evidenced by the reservoirs. Have no idea if they are original. Is there a pattern to who supplied what in the 70s? Also, I imagine Girlock would be manufactured later than Lockheed.

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Clive G
1969 Mk II Cooper S
1970 MK II Cooper S
1963 Triumph Spitfire
1982 RX-7 Series 2

My memory has it that Girlock was the company name that emerged from the merger of Girling and Lockheed.

I think it might be worth dismantling both master cylinders. If they have been stored for a while the seals may have dried out and would benefit by some rubber grease or brake fluid to help them to prime when you go to bleed each system.
Then you'll see the intensifier/non-return valve in the brake cylinder. They varied slightly in appearance from manufacturer. Some had a small metal dome held by the moulded rubber, some didn't have the metal part - just rubber with a small hole in it. This is from a PBR brake o/h kit.

Be aware that traditional brake fluid (non-silicone) is hygroscopic (water absorbing), so if brake fluid was used as a lubricant during assembly and they have been sitting for some time then you really should dismantle to check that all is well inside.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Here are my components. The size on both end rubbers is 11/16" and inner diameter is nearly .70". Guess they are both brake master cylinders. Both sets of internals look exactly the same.

Any comments would be helpfull.

Both of those have the non return valve and are setup as brake masters. Just leave out the non return valve from one of yours to turn it into a clutch master.

If you have clutch throw issues later you may want to look at a 0.75 clutch master sometime down the track. Or look for one now to prevent double handling.

Here are the internals for the clutch master.

Is the non return valve the one I have arrowed?

Yes

Here is a section through a brake master. Its a tin type but it shows the non return valve at No16.



I didn't have an exploded view of the plastic tank brake master so i just added the non return valve to this pic.