I was going to tell you to look at the check valves again next, but you beat me to it. Those stupid little check valves cause more problems. Well the dirt in the vac system could be from any little leak in the system (even one that is too small to affect the system). Because if there is any leak in the system, that leak is constantly sucking in whatever is outside of it. So if you drive down a dusty road, or through a big mud puddle and dust and dirt or water gets around that leak, its going to get sucked in. Even more so if you had all the lines replaced because of what im assuming could have been a leak.
2012 F-150 XLT 4x4 EcoBeast SuperCrew
2007 F-150 XLT 4x4 5.4 SuperCrew - Leveling Kit - Magnaflow Cat Back - Tonneau cover - Traded
2004 F-150 4x4 S-Cab, 4.6, Auto, Edge Programmer - Traded
Everything You Wanted To Know About IWE's