Losing brakes on gravel and grass

Intermittent problem with brake loss when driving on gravel or grass.  This
has never happened on paved roads and only occasionally on grass and
gravel, but when it does happen, brake pedal goes to the floor and vehicle
does not stop.  Mechanic checked everything and could not find a problem.  
Could this be a Traction Control/ABS issue?  If so, is there a way to
test/isolate the problem?

This vehicle will soon be my youngest sons first vehicle (new driver), but I
am hesitant to hand him the keys until I am confident it will stop EVERY time
when he brakes.