Brake problem

["Allan Weidenheimer" <allanw@domain.elided>]

My brake failure light came on yesterday.  And it has come 
on before.  But, it only happens when I make brake hard
while carrying a yard of gravel in the back.  During these
hard braking (neccessary because some guy in a rice 
burner wants to drive up my tailpipe on a rural 50 mph road)
I'm on the pedal hard, slowing to make a turn.  Once
the light comes on, it stays on, of course, until I 
remove it, apply the brakes once while parked and
put it back in.  Before, during, and after, the truck
stops perfectly straight, and it stops well for something
that weighs 8,000lb+.  Anyone with some thoughts?
With the truck empty, the light won't come on, no
matter how hard the brakes are on.  I understand
this light comes on when there is a pressure difference
between the front and back systems.  BTW, this is 
a 3/4 ton pickup, 12" drums all the way around.  It
is using silicone fluid, wheel&master cyls replaced 
about 4 years ago.
Some of my thoughts:

Air in front or back lines?
Rear rubber hose ballooning? (it's old, fronts are new)

Allan Weidenheimer
72 1210 Pickup