Something sounds fishy,
I too don't know why you would be given a wheel cylinder that uses a bubble flare fitting for your old Chevy truck.
Some wheel cylinders have an inverted flare and directly take a double flared line with a male threaded tube nut.
Others take a "banjo bolt and fitting" that then has the inverted flare for your line.
Others will be the metric bubble flare.
Not that it is relevant here, but a bubble flare "can" be approximated by using the first step of a double flare operation, but professionally done bubble flares are most often made with special flaring tools designed for just that.
Grigg
