Rear Drum Brake

After swapping out the manual clutch for an autoclutch on my Whizzer, I decided I'd be more comfortable with two hand brakes instead of the awkward coaster. I tracked down an old Atom rear drum brake (like an Arai) from a Schwinn tandem, installed it.....and......well, ....the stopping power is less than ideal. It functions, but seems mushy and weak. Since, I've juiced up the bike with various speed mods, the braking has become much more important. The shoes look adequate, the cable is taut, and I gave the shoes a light sanding to remove a glaze. I can, with effort, turn the wheel by hand when the hand lever is pulled. The front wheel is the stock drum brake and that one seems to work well.
In short, any ideas for improving this rear brakes power? Ideally, I think the brakes should be able to skid the bike.
Thanks in advance.

Does anyone know about getting replacement shoes for Atom drum brakes? There's a place in England that will reline brake shoes...but the turn around time is probably lengthy.
Thanks

Thanks, 2Door
I'm using a a full sized handle, though it didn't come with the brake. I think you're right about the physics issue.
I appreciate the ideas.
Best, Gery

Thanks....that's really helpful.