Flushing brake lines

Doing the brakes and the leftover fluid in the lines is almost black. I want to flash the lines before putting in new fluid.

Is mineral spirits ok for this? How should I pump it through? Use the mastercylinder?

 

no ! dont put anything but brake fluid through it,anything else will damage the seals.just bleed it thru with fluid until it comes out clean.brake fluid aint that dear.better yet pull the wheel cylinders out,clean em out and fit new seals.its cheap to do and the peace of mind is worth it.

 

As lowsquire said,if you're that far into the job you might as well pull the cylinders(master included)and clean them out and install new seals.Don't know what kind of car you're working on,but rebuild parts are usually inexpensive.
I use a pressure bleeder on all my cars.I bought it at a tool sale for about 20 bucks awhile ago.Came with an adaptor for twin circuit brakes(Ford style master).Easy to make adaptors for whatever you're working on.Just get a spare m/cylinder cap and drill a hole in it for a male air fitting and seal it.Tank on bleeder pressurizes at about 20 psi(after you fill it with fluid)and a line with a female air fitting goes to adaptor on M/cyl.Hook it up and start bleeding from furthest cylinder.When it flows clear fluid,shut it down and go to the next one til all are done.Mine was made by Blue-Point and I think Snap-On has an identical unit.Don't know the cost but it's a worthwhile tool.