Good suggestion Mike.
I am at Merle’s Auto Parts now, and it’s a little more complicated.
They don’t make a reducer that jumps from 1/2-20 directly to 3/8-24, you have to go to a quarter inch first. But that string of three fittings (two reducers, and a union) is a lot better solution than what I was considering, which was far from ideal, as the brass coupler was pipe threads (tapered) and would have left a huge air pocket that would have been very hard to bleed.
Here's the solution I am going with.
I also heard from a friend (Thanks, Larry!) who suggested that reducer fittings are found on Ford Master Cylinders. I had not turned any of these up, but if anyone has a part number, this could be a single jump to a 3/8-24 union.
".......if you were looking for a fitting that would reduce the 1/2-20 flare nuts to the 3/8-24 ‘normal’ sized ones, the smaller of the two brass adapters installed into the various Ford iron master cylinders will do the job."