3000psi from an n2 rig attached to the mast normally clears it!
Mind you it'll take a few hours to clean all the galleys out.
If only the CC didn't put milk and red wine and coffee grains down the sinks in the first place.
Worst I've seen recently on a 744 was a total blockage of around 10 feet of drain line in the fwd cargo bay, ceiling panels down, lots of pipes out and half in dia steel rod, just about cleared it!