changing brake fluid

[Gnasher]

Glad you got them out. 

Now check the tapered seat on the nipple if it's scored or damaged it's highly likely the taper in the caliper body is to.  It's this taper between the nipple and the caliper body the forms the seal, not the thread.  They get tight mainly through over tightening which stretches the thread and damages the taper seat in the caliper body, it being aluminium and much softer.  Over time owners and stealers nip them up more and more eventaily you can't undo them in the normal way and new nipples won't seal without over tightening and in short order you're right back where you started.

The amount of water the fluid will absorb while refilling and bleeding the system in your garage, will be undetectable this side of laboratory and even that it will be so small as to make no difference.  You'll be fine to fill and bleed the system, obviously if it's just too cold to work that's different  Be sure not to drain it, leave the system full and check the nipple/s periodicly if they start weeping the taper seats are shot.