The brightness of the bulb(s) on a ped can vary a lot with RPM. From dim orange at idle to bright at full speed.
I don't know how your taillamp is wired - some cut out the tail filament when the brake comes on, others light both filaments.
If it's a 6V system, a 12V bulb will only make things dimmer. If it's a 12V system, that 6V bulb would be about toast by now. The only dual-filament tail lamp bulb that you find in most auto stores is the 1154.. and they seem to be fine for 6V peds. At least they work in a Motobecane.
When you installed the new socket, are you sure you connected the wires correctly? Bulb case to ground, and the stop & tail wires to the correct contacts?