I have been running sythenic since I first put the Lister clone on the road in 1997.Yes everything was new except for the brakelight switch.It lasted a year.Replaced with a new one and haven’t experienced any problem since.Might just have been the switch was old and just failed.
It is important however that nothing of the old fluid remains in the system.This means blowing the lines absolutely clean or replacing them as I did.
As I said eveything was new except the switch and once replaced there were no more problems.Does this mean there is a reaction with the mixed fluids? The answer is I really don’t know.
A few people have gone to a pedal operated brake switch with great results.At this time I see no need to do so but who knows.