I need to purchase a metal brake pipe union fitting (male). It needs to fit into a female flare fitting that's 3/-24 thread size. If I purchase the male one that's 3/8" x 24 UNF 3/16 is this the correct thread size as below?
http://www.ebay.co.uk/itm/Brake-Pipe-Brass-Union-Fitting-Male-3-8-x-24-UNF-3-16-/120952753059?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1c29588ba3

That sounds right to me I think.... Not got any to hand to check

That's an unusual size these days Mel, unless it's for something old and British or American derived.

What's it for?

Its for a (American) Wilwood brake residual pressure valves (2lbs) that I plan to fit on front CNC disc brakes.

Same as an old Morris Minor then 

For the gp or the manx?

for the GP buggy