Bought a 1984 TheMightyHobie18. Rudder assembly/condition appears to be good. But.....port one won't stay locked down when underway, starboard one won't kick up when it touches bottom. Any help
Look on the Hobie cat web site under the support section http:/
I'm not sure when the style changed but the old ones didn't have the kickup rudders. My old 18 didn't and as a result a couple of castings and one rudder was broken over the years. There is a retro fit kit though. If it already has them, it is just a matter of adjustment.
Clayton
The original Hobie 18 rudder system was tempemental. It had metal cams. It was "Kick Up" but often would stick down. You have to keep them lubricated and there would often be a small ding in the casting that the roll pin would stick on. File that area flat. Lift the tiller arm when coming into the beach. If one does not stay down, the blade is likely drilled wrong.
Upgrading to the current Hobie 18 system would resolve all those problems. I had a Hobie 18 with that old style and it cost me a rudder blade when I couldn't get the cam to pop loose just before entering the surf....mind you, I installed the rudders backwards (port on starboard, etc) so the rudders were highly loaded making the cams stick even harder - that's why I was going back to the beach! 
I have seen the new rudder castings for sale on eBay. Some times they come as a complete set and other times as parts. Either way it is still substantially less expensive than buying new.