fuel tank standpipes

any of you guys that have had problems with your engine sucking air,(fuel lines sucking shut or collaping,cracked fittings ect.) ever find a crack in the standpipe in the tank. my 6nz has lost prime twice now in the last to weeks right after shut down. so i'm in the proccess of replacing fuel lines, fittings and such but really have not found any thing that looks bad. thanks in advance. cubie

 

If it was o-rings or a check valve would't it do it all the time. this has only happened twice and both times there was about 1/3 of a tank of fuel. I don't know maybe I' over thinking this. thanks cubie