Soldering to brass can be tricky for sure - you have to have the surface clean pretty good - isop. alcohol, maybe light sanding, also flux and heat is the key - I'll usually hold to iron on the brass for a couple of secs first when I'm trying to get solder to bond to it.
What's a PZL driver? I'm finding references, even a link that doesn't work (no direct post #). Links to posts don't ever work for me...
Oops - found the A17PZL driver thread: https://budgetlightforum.com/t/-/30227
I agree - it's a real nice host to mod. Cranked up, it gets incredible output for a 14500.