Carrier comfort pro hard starting

I have a weird issue with my apu. It doesn't want to start unless the main truck engine is running. Something odd happened when I pulled the fuel filter, fuel continued to drain down out of the filter housing, without stopping, as I replaced the fuel filter. Primed FF, replaced, and tried to start apu. It wouldn't fire. Tried several times, no luck. Ran main engine less than 5 minutes, tried to start apu, it fired right off and ran... shut main engine off. Apu ran all night.... one day later, tried to start apu... same exact thing. Will fire off and run if main engine is running, (less than 5 minutes main engine run time.) This is weird as hell...

 

Hmmm... I'm thinking fuel check valve likely bad.... would running the engine pressurize the fuel enough to push it to the pump?