When I first built my FE, due to readily available parts (on my shelf) I fitted an LC Booster, Master Cylinder, Distribution block/Warning Light and front discs with HR rear (disc type) drums.
I have since fitted LH front discs and calipers, it was all good up until recently when the brake warning light came on. I checked the wire connected to the Dist block, checked brake fluid levels (no change), there has been no dropped brake fluid or any apparent leaks. Braking has been normal apart from a slight squeal at low speed, I have not tried an emergency stop though.
Any ideas?
Sounds like a dodgy switch or earth.
If you disconnect the wire from the switch does the light go out, if not the wire is shorted to ground some where. If it does go out, try un screwing the switch and then re install it some types can be reset that way.
Often the distribution block needs a rebuild as the internals get stuck
Ken