I've been having a similar problem of the rear brake not switching the lights on soon enough but then a delay in them going out. Turned out the actuator wire was jammed alongside the tank breather and causing resistance. Having sorted that out, I would like to adjust the switch to come on slightly sooner but the switch body appears to have bound into the adjuster nut and I don't have the hands of an elf to get in and adjust it.
Task for another day I fear.