Declan
If you are having trouble finding the rear blanking plus for the Minor master cylinder have you considered the alternative of fitting a brake master cylinder that was designed for a remote brake fluid reservoir.
see the attached link for what is suitable for mounting inside the Minor chassis leg.
https://www.merlinmotorsport.co.uk/p/gi ... 1-gmc-10-r
The master cylinders are available in different internal diameters and the connections/unions are readily available.
I have a spare master cylinder of the type as described in the link and have made/taken preliminary investigations/measurements as to its suitability and fit and it does sit inside the chassis leg very comfortably.
It will need a front mounting plate which is easily made and would be similar to the one I have already made and is currently in use for the clutch master cylinder on my Minor.
It is my long term intention to fit the spare master cylinder (as per the link) at some time in the future to my Minor and which would replace the O/E type Minor brake master cylinder.
It would be also possible to fit a dual master cylinder to provide a split brake system if required.
Phil