Today was my first 'wet' commute since installing the new drum brake and lights, and the brake worked like a charm. The rims on the Schwinn are old and I believe they are stainless. Whatever material they are, when they get wet the brakes don't work. Last winter I got to school with numb hands and forearms from gripping the brake levers with all my strength to try and slow down. Basically, I had to hold the pads tight enough to the rim to dry everything off through friction/heat and then the brakes would catch. When the rims are wet from snow slush you can imagine how long it takes to dry them in that manner.
It's not actually raining today but more of a heavy mist. Heavy enough that I had to stop at a light and pull out my goggles so I could ride without squinting. Since it was a light traffic day I decided to test out the new brake. On the first hill I tried to slow with just the rear (caliper) brake and it slipped as I expected. When I tried the front it worked great. It was a little pricey to get the generator hub with the brake, but it was well worth it. Anyone building a new wheel for use in bad weather commuting should really consider using one of the drum brake hubs. They're common on tandems, which means the hubs are out there.