I have a 79 which the cruise didn't work when I bought it. I replaced the transducer, and now it will work if I hold the button in just a tad. Is there a rely or something that's supposed to engage to keep on?

So you're saying that it stops working if you take your finger off the button?
Well, if that's the case it could be some problem with the resistance wire that keeps the solenoid open. The top of the transducer should have two connectors on it. One of these is a resistance wire with a small current running through it all the time as long as the key is at "run" The other one is a normal wire and only provides current when you put your finger on the button. This way, when you push the button half in (to the set position) you get a big current to the solenoid inside the transducer, which "pops" it open and starts the cruise. Then when you release the button, the small current from the other wire is just enough to keep the solenoid open. You need a big "shove" to get a solenoid open, but only a trickle to keep it there. Seems your resistance wire is faulty or its connection to the solenoid is bad. Check out http://www.nottingham.ac.uk/~eaztl1/Cruise_control/Cruise_control_troubleshooting.htm, my trouble shooting page.
Good luck with it!