93 YJ sudden brake fade

93 YJ stock brake components no ABS.
Brakes have 'suddenly' (week and a half) started to 'fade' -
If I try to stop suddenly the brakes do not lock up AND the brake light
will 
come on, tapping the brake pedal makes the light go out - stopping
distance 
is noticeably increased. Can't find anything in the Chilton's on this.

Reservoir fluid level is good. It was a little low last week (3/16" /
4.8mm 
from top) but topping it off has shown no different results.

It is going to be above freezing this weekend (finally!) and I will be 
taking the tires off to look at stuff - any ideas as to what I should look

close at and what can be disregarded?

tia
paul