I de-linked the LM3 and the LM4, both with twin F08 calipers. On the LM4 I used the original 12mm master, and would have on the LM3 except the casting broke. It now has a newer master with the rectangular reservoir. It's also a 12mm, which is actually closer to 13mm being an imperial 1/2". I believe all brake parts are produced in imperial sizes only, then marketed as metric equivalents by companies dealing in metric parts like Brembo.
It will cost you nothing to try using the current master. If you don't like it, I'd be happy to buy it from you.
It is important eliminating all of the air or there will be excessive travel.