I've posted about this before. For about $15 in parts and materials, the old-style castings can be made to function properly. I use them on my beach boat sailing in and out through the surf and they kick up as intended.
First, the cam spring has nothing to do with rake or slop in the system. The cam spring tension simply determines how difficult it is to pop up the rudders (usually set to "very difficult"). The old style rudder system is a fixed rake system, so the only way you can adjust the rudder rake or take out slop is by re-drilling the holes in the rudders. It's a more time consuming system because you have to fill in the existing holes with epoxy, mark, and re-drill the new holes so that the rudders are raked where you want them. Then sail the boat and verify the helm is good. If it isn't, you repeat the process. Check the recent Hotline articles for more on how to do this.
In my experience, there are two main issues that cause the old style rudder system to malfunction. The first is lack of lubrication, and the second is over-tightening the cam spring. I suggest using an automotive bearing grease to lubricate the system. If you look, you will see where the roller in the upper casting contacts and engages the hook (cam) in the lower casting. Apply a thin coat of grease to these surfaces. Don't go overboard or there is potential to attract a lot of dirt/sand. If you sail a lot, you will need to clean out the old grease and re-apply a couple times during the season.
The next thing is you need to loosen the cam springs. That means probably drilling out the frozen plastic screws and replacing them. Set the cam spring to the absolute minimum tension so that the upper casting just barely clicks into the lower cam. One of my screws has a piece of duct tape around it to keep it from unscrewing, that's how loose I run the springs - just enough to keep the upper casting from accidentally popping up. Then use a piece of 1/4" bungee wrapped around the rudder blade and rudder pin (twice). This is what is primarily used to hold the rudder down while underway. Install a pair of "Kisme Kleets" on each lower rudder casting to hold the bungee in position.
The pressure from the bungee cord will also help to correct any slop in the rudder system and keep the rudders pulled forward so you don't get excessive weather helm.
After several broken lower rudder castings, I evolved to this system and it has been working successfuly for well over ten years now, including when sailing up onto the beach through the surf.