Hi,
Thanks for all the help.
I followed Andy's advice and did a practice re-tap on my metal metric male brake union. This went well so tried it on my builder solutions supplied bulkhead metric connectors.
Unfortunately, I'm not sure if it was my incompetence or the softness of the brass material causing the die to follow the original threads, the first one was perfect, the 2nd and 3rd ended up with a non straight thread.
As the car is a Southern GT all of which seem to be built by perfectionist experts, I thought as a confirmed botcher I had better at least try and get something right.
So as builder solutions supply a 3/8 UNF bulkhead connector I have decided to get these and just replace the metric connectors on the brake pipe side.
Once again thank you for all your advice much appreciated.