I think the threads are UNF threads. The seat is past the threads a little way, so if you don't nick it and use a bottoming tap for the last few turns you should be fine. What size brake cylinder is it? If it is the same as a 1950 huck cylinder I have a set of decent cores I could part with, ping me offlist. I just removed them from my truck, and I can hone them for you. You will need to kit them. To check to make sure on the threads, buy a new bleeder and check the tap against it. Those bleeders can really stick in there. Be careful using heat so you don't warp the cylinder.
