near empty tanks cause other sorts of problems, but not leaks. If the macro is properly sealed up it shouldn't leak, and if it isn't set up well, it'd leak at the macro fitting and not the back. Your problem is orings. Orings should be bought online or from reputable paintball stores. Here's a pm6 oring kit for freakin cheap
Captain O-Ring - PM6 Dude's got very good customer service too.