I would have thought the adaptor would seat into the bottom of the M12 hole like the original brake line. Did the adaptor have a taper on the m12 male end and does the M10 female have a taper at the bottom for the brake line flare to seat into? - something like the attached. If so you should be fine.
Unless it is going to seal at the bottom it won't matter how tight it is - hard to tell without seeing it. You could try a washer but the application is not strictly designed for it. You may need to shorten the adaptor to make sure the washer is sandwiched enough (obviously needs to be some sort of squash/seal washer). Good luck.@Camsmyth
Thanks mate. male end is not tapered, though female end is
View attachment 1344365
Now I can do the adapter up pretty tight though and it wont budge, but Im just paranoid that it will either come loose or bugger up the inside of the mc if done up to much
I dont like to do these things on the car without having complete belief in what im doing, even if its silly things
is it just the pic or is that a tapered thread?
In that case as long as the taper is working before it bottoms out you'll probably be ok. I normally use sealant on tapered threads and find they degrade if you remove/refit too many times.@bloke
It does. I think i have answered my own question as the adapter does flush via the slightly wider tapered end rather than in the bottom of the mc. Was just not apparent it was!