Did you try back-flushing, like Dave S. said? Go to the hardware store, get a 1/4" pipe nipple and an adapter to a garden hose. When back-flushing I like to rig so that I have a valve close to the truck so I can see what's happening.
You could also hook up to an air compressor, if you have one. The threaded end of an air hose is 1/4" pipe thread, too.
