What year are we talking about??....there is brake lines available with metric threads for both styles of flares,the typical double flare and the metric "bubble" inverted flare fittings--they are 10MM and will "almost" thread into a standard fitting that has the 3/8"x24 threads used on most 3/16" lines but they will nit tighten and will strip the threads--how do I know this??--I ruined my rubber brake hoses when I screwed one of these metric threaded brake lines into my hoses!--Auto-Zone had the brake lines all mixed up and some metric ones were mixed in with the "standard" ones,they had the correct flare but the metric threads!...I brought my brake hoses in the store and told the manager it was partly MY fault for not loking closely at the lines before I hastily installed them outside in snow lying on my back--he said they were at fault for not having the correct lines in the right place on the rack,and he gave me two new brake hoses at no charge,which I appreciated,but I had to wait two days for them to come in..
GM uses many strange sized ports on things like master cylinders and brake proportioning valves,they often have a 3/16" or 1/4" brake line that has to screw into a 9/16x18 thread port,or a 3/16 line that needs a 1/2"x20 thread nut fitting like a 1/4" one has on it..there are a bunch of "adapter" fittings available ,but its rather difficult ti get the correct ones sometimes,and they are not cheap--so I often save the original line nuts by cutting the lines close to the fitting and use a 6 point socket to unscrew them and install them on the new lines ,rather than hunt all day for the right adapters and waste money on them and gas chasing them down..
I tend to use rolls of bulk tubing more than "ready made" hard lines,as the ones already made up are seldom the correct length or have the right fittings on both ends and I'd end up cutting and re-flaring them anyway--the bulk tubing is much thicker and corrosion resistant than the ready made lines too--your lucky to get 2-3 years out of a ready made line here if you drive in winter at all...
I had to replace the hard lines in my 1995 Ford Contour a few years ago in the rear,and I found it has a oddball 11mm line nut fitting at the brake hoses,and I had bought 3/16" lines with the right "bubble" flare ,but they had 10mm threads--had I not had problems with my trucks brake hoses before that ,I never would have checked closely,and I would have ended up stripping the threads in the brake hoses on that car too!--I had to save all the old line nuts ,cut the lines and re-flare them with the original fittings to get it back on the road...none of the parts stores had the Dorman 11mm to 10mm adapters in stock,I'd have had to buy a whole box of them special order to get them too...with all the different styles of brake line flares,fuitting threads ,etc,I am surprised a lot more cars aren't having problems when the wrong ones are installed..this is one area in automotive construction that should be "standard",and not have 20 different types of fittings!...I wish they would also make it mandatory for factory brake lines to be stainless steel in the rust belt states,and fuel & tranny cooler lines too--far too many acidents get caused by rusted brake lines failing in panic stop situations,and countless trannys have died prematurely thanks to a rusted cooler line letting all the fliud escape and I bet 90% of all car fires are caused by rotted steel fuel lines--the other 10% were likely set on fire intentionally!..
