Before you do anything, make sure the front system works! It should normally be nearly impossible to lock the rears with everything ELSE plumbed correctly and working properly.
he mentioned having a 318 with a 8 3/4 rear end and 10 inch drums
if that rear end is not original to the car, the brake bias might be off
add to that the possibility of larger, wider rear tires (with a much greater contact patch) and it may very well be possible the rears lock up before the fronts do
(thats the way my 71 with 9 inch fronts acted when i swapped in an 8 3/4 with 10 inch drums, at least untill i upgraded the fronts)
