i siphon gas out of junk cars as well. i put a fuel filter on my gas siphon kit. i may later put a cheap eletric fuel pump in its place later with a switch to turn on and off with.
what i nomaly do is take a fuel sample. if it clean fuel then the rest gets pumped into the gas can or truck or whatever. but if its real dirty,gummy,funny colors then i will not use it and not bother pumping it out.