Sorry, I should have made it clear that, while the reservoir/can is a brake item, the internals are specific to each use......

Cheers, Ian 

A brake master cylinder is .700" and a clutch master cylinder is .750" as a general rule for single line MCs.

The physical bodies of the two masters are identical aside from the angle of the can - the holes and passages are all in the same place and the same size.
However...

The actual internals of the piston arrangements are subtly different.  A clutch MC can be used for the brakes, but without the slow-fill valve you will not be able to double or triple pump to regain a fading pedal.  A brake MC used for the clutch can incur engagement issues as it may not engage nicely.

I spent a long time talking to AP Caparro (the manufacturers) while chasing down braking issues on Gertie.

I can't speak for what you have (obviously   However, I have used two brake masters for years.  The 90 deg offset does make it a VERY close fit with the float bowl of my twin 1.5s ..but it does fit.

The extra volume of the brake cal (0.75 bore) will help with the usual Mini clutch travel problem...

Cheers, Ian

First off I am aware that they are different. I know the reservoirs are off by almost 90degree. And i am sure the bores are different size. But I can not find what the bore on the clutch master is. I have fitted and modified the single line brake master in place of the clutch master as I MUST use the car this weekend and the newclutch master won't be here until Wednesday. Just wondering if I really need to change it back.