I'm not familiar with a Soda King, but if there are rubber/plastic gaskets in the head and they appear dry, a bit of lube may be in order.
I have found the use of lube when kegging beer has solved most small CO2 leaks.
An example of food grade lube can be found here:
http://www.northernbrewer.com/brewing/lubrifilm.htmlThis type of lube can also be found at many local stores which carry homebrew supplies. (I would call first)
Ron