Master cyl up grade 99 yukon

[skychair]

This may well be a really stupid Question, but
What does the 9/16 or 1/2 refer to when a brake line is 9/16-18, or 1/2-20?
Is it the nut size or diameter of the the threads?
I'm ready to do the Master Cylinder up grade, but the adapter I have BLF-26C will not screw into my Dorman M630031 master cylinder.
I dont understand...
Need some help/responses please.
Ron

 

[owlsarepretty105]

Unfortunately, I can not help you! I'm not good at this. Sorry!((