I have a 89 cherokee 4.0 with power brakes. Replaceing the master cylinder and couldn,t get rear line off so I cut it and now I can,t find that line at the store. Looks like 5/16 thread with the small line 3/16 .I don,t know my pipe thread stuff very well. It,s the short line about 6 inchs from master to proportion valve

sorry messed up fitting measures about 7/16 outside dia.

It is a standard brake line size, 3/16" The thing is that it has to have double flares on both ends. In a pinch you can buy a preflared line and bend it around to fit. Don't kink it though

If you get preassembled line make sure you get the proper thread. Metric or U.S. I don't know what it takes.

it,s SAE but I can,t find a 3/16 line with the larger fitting on the end to match the master cylinder and P. valve in the pre made lines

I believe the fittings at the master cylinder and p valve are 5/16. 2 of these would work if they are.
https://shop.advanceautoparts.com/p/...351/10318556-P

perfect, that will work. I,m in Can. so lots of stuff in the U.S. are not here especially when you start getting into the older stuff , but Dorman stuff is easy to get. Thanks as always.