thanks Geof
you are very good at saying in 1 line what takes me 20!
when i first wrote all that I was thinking that it was acme which is a wide cut (lots of cutting force) and that being in a tough material it was too much for the lathe. if you are using a bigger lathe, and its a V thread, you can ignore a lot of what i said.
I still think the problem could be in the grind. Check the side clearance on each side - what might work perfectly for RH, won't for LH - for RH you need no clearance on the right side of the tool, for LH you do - greater than the helix angle.
you can still us the traveling steady, which i suggest you do for a long spindly part like a feedscrew. Reverse the lathe spindle direction and turn the tool upside down, now you're cutting a LH thread from tailstock to headstock!
I'd use something tougher than brass, ie bronze. just make sure the tap is sharp. leaded freecutting is not the best material for the job, but it makes the job eminently doable and may still give a lifetime of service in a hobby lathe.....then again if have access to the bigger lathe and its only a v thread, you could try pre-hardened chrome moly.