usually when the brakes apply themselves it is because the air pressure gets too low, most trucks have buzzer that turns on when the pressure drops below 60 psi so you still have some time to stop and wait for the pressure to build back up. you get used to them after a while.
It sounds like your '72 has Maxi Brakes (Spring Brakes). My '71 9500 had Maxi's. I doubt very much too that the '66 would have Maxi Brakes. Not sure exactly what year they started showing up, but I believe it was somewhere around '68 (in Chevy's and GMC's anyway).
Bill
